In addition to all of the above things, some details like when an upcoming trip is planned, quantity and ages of children, etc, can be very telling.
Another scenario I've seen is someone who posts some things that come off as controversial/judgmental/etc, and get called out for it. I don't even mean being flamed unfairly, I mean legitimately called out. Then, all of a sudden, roughly a day later, a brand new poster appears asking an innocent sounding question about "do you ever great treated unfairly on these boards, becuase I've heard from some others that they have". It doesn't take much to do the math and figure it out
Overall, though, yes, it's just details about what a person is up to, what they do for work, their interests, that after enough posts (by the original username) can pretty clearly identify a person when they reappear.
ETA: Contrary to what some might suggest, it's worth noting that I believe most people on these boards aren't out to stalk people, or reserach them in great detail. It's just that when you converse with the same people enough, you start to get to know them. If anything, it only makes the interaction that much fuller, and helps make the discussion that much more useful. (For instance, remembering that they have a 12 year old son when they post asking about what resort to try next, helps one to give a better response.)