See this MSO post asking why the OP got the 100 points. The answer:
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.
Your first reputation points have to come the old-fashioned way (asking and answering). But once you know what's going on on an SE site (and we know you're not a troll) then you get a helpful initial boost for other SE sites.
This has been the case since around April 2011 (source).