I've used Youtube since it came out in 2007 and never seen this. It could be that either someone has your password, or you're visiting some site whose malicious Javascript is cross-site-diddling into your Youtube session. (However, search as I may, I can't find anything that would validate this latter hypothesis by hinting at the existence of such a malware.)
In this thread:
https://productforums.google.com/forum/#!topic/youtube/YRoOv...
a user persistently claimed that YT was subscribing him to channels just due to him viewing a sufficient number of random videos. At the end (scroll down) it turned out not to be the case: all the channels he was being subscribed to belonged to users that he thought he unsubbed, but just didn't unsub properly. The user accepted the answer about unsubscribing correctly.