If one of your followers deletes their action (e.g. un-liking a post, removing a comment, etc.), a negative social action will result.
For example, say one of your followers liked a post yesterday, but un-liked that same post today. You'll see -1 likes when viewing today's stats. However, if you broaden the time range and look at the all-time data for an content item, the total number will never be less than 0.