Half an hour ago, I was awarded a +100 association bonus on Math.SE. I don't know why. I have only one other StackExchange-account, an Area51-account that I have never really used (its reputation was 1 until I visited it fifteen minutes ago; then it received an association bonus from Math because my Math account has over 200 reputation, so that my Area51-account now has 101).
There seems to be no reason for me to receive the bonus, unless you're supposed to get the association bonus also on the account that has over 200 reputation. Even then, I don't know why I received the bonus today, since I have had over 200 reputation on Math.SE for some time.
The situation is similar to I was prematurely awarded an association bonus, but that issue is supposed to be solved.