I've been reading along and I hope people don't mind me jumping in...
Well, I think for many INFJs, the doorslam ends up happening once the INFJ has given up on the relationship improving. It could be that they've concluded it takes more energy than the results it yields, it may be that they've lost faith in the other person's good intentions, could be that their visions are so different that they don't believe that aligning them is possible, could be that suddenly past patterns come into focus as having meaning that they had not previously recognized, they might feel they are not going in the same direction as life, or they just are hearing so much emotional noise that they need distance, they may feel like their voice is not being heard, no matter how they try to communicate. They also may feel publicly embarrassed or betrayed by someone they had trusted with the inmost parts of their being. Or they just use doorslamming to avoid uncomfortable feelings or because they are not willing to put in the work of communicating.
Yes I think this is right, from my perspective.
I'm not really trying to define it from someone else's POV because mine is the only head I live in. I have access to some of the possible (perceived) motivations that I (or someone like me) might consider doing so and the reason I would offer those is in an attempt for the other parties that have come here to make sense of their situation. I can't right the wrongs that have happened between them and their significant other. Neither am I in the habit of doorslamming.
OK, I'm help PB out here and try to distil this whole balagan down (@mane will understand what I mean by that

).
So I think what element the INFJs may feel is missing from the "doorslam" definition is the way it reverberates (for the lack of a better term). I think when the definition is offered, all the INFJs can imagine is the hypothetical INFJ mindset behind it, which (naturally, like any type would) directs them see it only in a way that justifies those actions. In other words, you're only seeing why the INFJ would do it, not seeing it in terms of the back and forth of the Fe interpersonal system. Therefore it's difficult to imagine how it could be inappropriate reaction to a situation - am I correct?
Assuming I am, here's the way I see it...
I define the doorslam as a total mental/emotional/physical shutdown from another person. It happens when the INFJ feels they have repeatedly tried to remedy or alleviate a point of, or series of, conflict with another person. Eventually the INFJ feels that their emotional investment in this person is not of sufficient value, that their relationship is no longer worth preserving, and that the only way forward is to cut their losses, breaking all contact. Once this point is reached, it is next to impossible for the other person to re-establish contact and overcome the divide, because the INFJ's heart and mind is no longer open to engaging that person.
OK, so that's just a definition of the surface elements; the analysis (well the NP analysis, anyway) of the 'meta-intentions' are slightly different.* The underlying reasons for it are, as has been mentioned, are perhaps more to do with self-preservation and reclaiming a sense of self. The INFJ might think of it as lifting a dark cloud that has been bearing down on them, but perhaps it is really an attempt to shut out undesirable reflections on them. I mean this in that, Fe uses other people as a kind of mirror to their own thoughts/feelings/beliefs etc. They send out ideas/information and they like to have to bounced back to them, in a slightly filtered form. That way the INFJ can see where that other stands (emotionally, mentally), verifies where they stand themselves, and details where they stand in relationship to one another. (Am I correct in saying this?) But when that other person constantly reflects back an ugly image (through conflict and/or failed interaction), they begin to impose a negative identity on the INFJ. When the input doesn't achieve the desired/expected output then it appears (to the INFJ) like an attack on the validity of their input. In this sense, the (perceived or real) toxicity of the relationship begins to undermine the sense of self of the INFJ. It threatens their understanding of themselves (what sort of person they really are) and of how the world works (how to read human behaviour and how successful interaction works). When things go too far, they shut out those negative reflections of themselves in order to overcome this situation. I can see why this is wholly justified in their heads - it makes sense.
What others have been critical about is the fact that (in their eyes) the INFJ is simply running from things; that they're turning their back on someone simply don't like what they're hearing and don't like how it makes them look. It seems like they're closing their eyes, and covering their ears when someone else draws attention to their own flaws too much. It is a refusal to own those (possible) flaws and is a denial of their own (potential) role in creating the conflict. It seems like a way of making themselves into the good-guy and the other person the bad-guy, whilst denying the possibility of anyone ever revising or rectifying this view.
I want to emphasize that I think the "doorslam" is not automatically a negative thing. I see it as a pattern of behaviour, a way of dealing with certain kinds of interactions and the feelings the arise from it. I think in certain circumstances it might be totally justified. I agree that it probably doesn't happen often for most INFJs - it's typically a last resort. I think perhaps, that what the NPs are also discussing in part is the fact that there are lesser forms of it (and are combining the two ideas in together). Sometimes, it might not be a total shutdown but a sort of "block" that sometimes comes down, that prevents INFJs from truly, openly engaging with the content offered. It feels like you're speaking but they're
not really listening, even though they may be replying. On the receiving end, it can feel much like the sort of arrogant dismissal of one's argument/belief/ideas that INTJs can sometimes engage in - like when they can't just disagree with you, they must make you feel small and backwards for thinking differently to them, and in return make themselves superior. It's like your, potentially quite viable/justifiable, alternative view of things is so threatening to their own, that they must dismiss it (and you) utterly and completely in order for them to feel secure. I emphasize that this is just how it feels, and does not necessarily reflect on what is intended.
*I'm going to offer a possible explanation, so forgive me if I'm wrong - I'm just trying to nut it out and don't mean to sound presumptuous. Feel free to correct me or make alterations.