I was just awarded +100 reputation on all of my Stack Exchange accounts.
What is this bonus for? It simply says Association Bonus on my reputation overview.
Congratulations! One of your linked accounts has reached 200 reputation points, and as a result, all associated accounts are given a 100 point bonus.
The bonus is awarded because you have proven that you know your way around the basic features of any Stack Exchange website, and with those 100 extra points you can now comment, vote, flag and create bounties on all SE sites.
From here on out, on any new sites you join you'll also be given this bonus.