YouTube does so purges of channels in order to remove dead / ghost sub account, also they may be some natural decrease throughout the month as users may decide they no longer wish to be subscribe or may have even become bored / disinterested in the content. At the end of the day there could be 101 reasons why someone unsubscribes (wish YouTube had a feedback feature) but this is all part of the 'Youtube' experience so please don't get disheartened by this