There is a current known issue with the Twitter API that is causing random tweets to become spontaneously favorited, even if they have not actually been favorited by the Twitter user.
This issue affects all Twitter clients using the API and isn't unique to TweetCaster. Twitter are aware of this issue and there is an open ticket on their API issue tracker here: https://dev.twitter.com/issues/4
They believe this issue to be mostly resolved at this time, but there may be sporadic instances.