I've shut people out of my life "suddenly" in two contexts
1.) A friend has done something underhanded I would consider the ultimate betrayal of trust that has at the same time caused damage to a meaningful relationship or my work.
2) Someone continuing on with a repeated harmful behavior that I've mentioned several times is an issue I cannot live with (like for example talking to me consistently in a derisive way for example, or other types of toxic behavior). In these cases I take into account their life situations and other "stress factors", but if it's just ongoing for a long time there can be a cutoff point where I just won't put up with it anymore and I'll leave the relationship. But if this happens I've discussed it enough times for them to have had plenty of opportunity to make good. Conversation after that is moot because it's been conversed a billion times already.
That said I'll weather huge amounts of crazy behavior from certain people who are permanent fixtures in my life. People I know have their heart in the right place...(unless they take it TOO FAR and that would be far indeed... like setting my house on fire or something...see no 1 or in intimate relationships 2. although I never sever the ties completely. They just get demoted to friends if they want to stick around. I don't see this differing from normal break-ups other types have...except I'm on good terms with my Ex:s )
It takes a long time for people to become what I'd consider "friends" before that they fall into the colleagues or acquaintaces categories and though I wouldn't shut anyone of them out with no communication unless they'd do something inexcusable. If I'm not that interested in a deeper friendship I wouldn't keep contact necessarily, but the phone works two ways and I do pick it up. But this is what other Feelers do too. ISFJs ENFJs ESFJs spring to mind... ...then again ESTPs are polite too even to those they don't want to be around...so it's not just INFJ domain that there. Some more insensitive types just say "I don't want to hang out" or something. That's their perogative. I kind of fade gradually if I don't want to be friends with someone. If they really just don't take the hint (like me not being available for anything for a half a year...) and they keep calling and pursuing I'd at some point stop answering the phone. As would other types. Sometimes people misunderstand my communication style and read too much into it. Maybe then they feel upset, but that's a two person miscommunication, not all my fault so to speak. Like ENFPs being warm and personable all the time or ESTPs seeming interested in you specifically for example. It's a style. INFJs have a style of being friendly and attentive in communications. Don't read too much into it.
...so to those who've been "doorslammed" by an INFJ here's some questions (because there's not really much backstory wise there either. Just people saying INFJs have doorslammed you, because they tend to do that...which sounds weird to me) :
IF it's a long term close friendship:
Has there been an incident where you've betrayed their trust? That's something INFJs really dislike with a passion. Have you cheated on them? Been flirting with other people? Told things shared in confidence to others? Used their ideas without permission?
Have your actions resulted in problems in their other close relationships? Or jeopardized their projects? Have you thought you could do things behind their backs? Have you been an all around inconsiderate boob souring atmospheres left and right causing problems for people (not that you'd necessarily have sufficient self awareness to realize it)?
IF it's more casual have they been aloof and distant otherwise too. Have you wanted the relationship to be more, very strongly pursued and they retreated? That means it's most likely one sided interest on your part and you're putting them in an akward situation of having to spell it out for you... though if they door slam you...are you what could be termed "a stalker"? Do you call a million times a day? Do you follow them around town? Do you imagine relationships with them that they are "too blind to see"? Stalker. I've doorslammed one. I think it's called self defense.
Those would be things that would upset me.
One thing you've got to remember is that INFJs can smell out inconsistencies in behavior and intension and all types of underlying nuanced motives people tend to have so when some (especially strong Se types and maybe strong T types) don't see there being any problems in a situation the INFJ would be (at least one of) the first to know. ...and us INFJ folks don't like to live in these kind of tense states. It's not good for our healths.