Q:

100 Reputation bonus upon joining new Stack Exchange site

Why is a 100 reputation bonus awarded for logging in to a new Stack Exchange site? 
This occurs if if you are an existing user of SE with some reputation on other sites. 

+100
You've earned a bonus of 100 reputation because we trust you on other sites in the network

A:

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.  
See Award account association bonus automatically upon reaching 200 rep
Originally posted by yours truly on Meta. SE: What is the association bonus?