Several factors may play into this. Insecurity, Conflict aversion, a wanting for approval, etc.
Type wise, of course 9's are the most obvious type to pin the yes man label upon(along with 2's) based upon the stereotypical perception of 9's as abhorring and going to great lengths to flee from conflict. However, I could very much see 4's being caught up in a yes cycle, after all they have a line to 2, which could potentially make them very "help-y", social 4's especially. Really, social doms in general as I don't think social lasts (or even social seconds though exceptions may be present in a select few social last/second subtypes, most namely sp 4's who's tendency towards "suffering in silence" could be conducive towards certain aspects of self erasure) would be so much inclined to have trouble with this, especially not in pertinence with the third possible factor mentioned.
As for 9's, I can't see Sx 9's(especially 9w8's) falling into this particular trap being that they usually possess an almost inately stronger connection to 3 than say, social nines who's connection to 3 may actually not so healthily manifest itself in people pleasing behavior. I also couldn't see sp 9's doing this either being that from personal experience(as well as what I've picked up from a few of the comments I've seen from other sp 9's), helping others seems too much a bother and I'd only do so if I felt I would need a favor from the individual at a later time. I find saying no comes too easily, and saying yes to be very hard as I hate going out of my way and discomforting myself for others, generally speaking.
Succinctly put, this is far and away from exclusively being a "9 thing".
