This has been happening to me and to many others for months now. 

Here's an active link talking about how subscriber losses are offsetting subscriber gains.

Sharing and Connecting on YouTube

"We aren't just losing invalid or idle account SUBS - There's a glitch; I've proved it. 

https://groups.google.com/a/googleproductforums.com/forum/#!category-topic/youtube/sharing-and-connecting-on-youtube/V4DPLGCzp84