Fluctuation in your subscribers count is normal. We regularly remove spam subs or it's probably viewers unsubscribing & subscribing to your channel normally: https://goo.gl/croiE1 . You can also review info on Subscriber count and Subscribers report from there. Hope this helps.
YouTube doesn't automatically unsubscribe users. We'd recommend checking this Help Center article, under "Does YouTube automatically unsubscribe users from channels?" for various FAQs and to understand how subscribers work: https://goo.gl/1FsxZ2 . Hope this helps clarify.