Why does my mouse sensitivity spontaneously change?

This appears to be a known bug that Riot is working to fix (click the Riot logo next to "First Riot Post" in this link):

http://na.leagueoflegends.com/board/showthread.php?t=2721861

A few people have reported it fixed as of the last patch, so chime in on that forum post if you're still having trouble after patching! And make sure to submit your logs any game the sensitivity bug occurs.