The only explanations I can think of are (1) altruism, (2) intent to profit from one's reputation, and (3) simply enjoying puzzle solving. Am I missing any? And are any of them so strange?
(2) And (3) aren't strange and are very likely the main drivers. It isn't hard to imagine that humans, and really any intelligent life, have evolved to enjoy problem solving and structure incentives around being good at it.
For me it is contributing to a project that has helped me too. For every useful answer I received on stack overflow I've answered some questions myself.
Interesting. I wonder whether "repaying a debt you feel but are under no legal obligation to repay" should qualify as altruism. (It feels altruistic to me.)