It just happens. The person is probably actively trying to help him/herself. I recall joining a forum, starting a thread about a medical issue (of my guinea pig), and forgetting about it while focusing on other information sources.
Months later, when the issue was long handled, I recalled about that and decided to go back and share the solution/conclusion. People there were already providing pages of help (and lots of off topic discussion too, but that's beside the point).
New members may join from a momentary impulse and will soon forget the forum's URL.
