It seems the game is still considering you to be in combat which is why you cannot mount. I am guessing at that as very little information has been provided. Regardless, it would seem you have not be in Cyrodiil or visited the forums in awhile as that bug has been around.
This is more of a "How is this still happening?" post rather than anything else. I am a frequent pvper. There was a fix implemented a month or two ago that said it would help with this issue.