You're not the only one losing subscribers. My friend went down I think by about 2,000 subscribers lately. Maybe more, I don't have over a thousand yet but went down 20-30 subscribers, so I think they're purging inactive accounts or something. I think they did something like this before. I could be wrong though since this is just a hunch.
Nigahiga, Smosh, RayWilliamJohnson, and other popular Youtubers aparently has also been losing thousands and hundreds of thousands of subscribers too. It's probably that Youtube has begun deleting inactive accounts again but also involuntarily making some accounts unsubscribe according to some reports.
That'd make sense. At least 75% of my sub count is subbed to me for no reason whatsoever, judging by my view count. Heck, I think even Chugga lost thousands of subscribers.
In addition to unsubscribing me to half the channels im subscribed to, Youtube has also subscribed me to about 20 other channels I have never even seen in my life, with most of them being Youtube's new "topic channels." This is, of course, after they break the "more videos" button on the homepage. I'm telling you, this is becoming more and more like communism by the day.
I lost 30 of my subscriptions recently, but they were all actually inactive accounts; I didn't lose one that I could remember. Only even checked because of your helpful video. Ha!