Right now I am feeling slightly angry about this: A two-part question was posted and an answer to both parts added withoin less than an hour. Nothing happened for three weeks (no comments or request for further explanation, no additional answers. Then, a few hours ago, a first comment appears (which I could only answer a few hours later) and shortly after that a self-answer - for only one of the two parts. The asker has the right to accept whatever he likes, but I find this somewhat strange and don't know how to react ...
This isn't really even strange behaviour by the community at large; it's just strange behaviour by one particular user. If it were the former, we could get together and discuss it here and try to agree on norms.
But this is a case that happens every once in a while, when one comes across users whose idea of interacting with the site is what one might see as... suboptimal. You can of course try to engage with them in the comments; they may realize they accepted their own answer prematurely and try to be more responsive to other responses in the future. Or it might turn out, as has happened to me before, that they just don't care to listen to your suggestions -- in that case, there's nothing to do but just shrug it off and move on, and perhaps avoid investing time into answering their questions in the future.