Okay so there is definitely something wrong with YouTube. Lost 4 subscribers in 1.5 hours since uploading my not a booktube newbie tag. Yeah somehow I don't think people were that offended by it that they had to unsubscribe....
We've confirmed there are no related technical issues for sub counts. It's likely folks subscribing and unsubscribing naturally. Explained in detail here: https://goo.gl/6B1Eb6 . Hope this helps clarify!