There has been a bug since Morrowind where if you use a wayshrine to escape combat you stay in combat until you relog. They mostly fixed it, but it definitely takes longer to get out of combat.

I have noticed it happens usually in 2 situations most often:
1) You kill something so fast with a skill that it doesn't quite register properly, often while it may be summoning help, or just like a neutral mouse or deer with a single light attack. It was dead before you entered combat so you stay in for 10 seconds to make sure you don't get hit or trigger combat again, as if you ran away.
2) You have a pet that somehow still stays triggered the same as above.

It seems to be because the combat status check happens after 10 seconds because of a final hit that occurs at the moment of, or because of lag slightly after, combat actually ends which it considers another fight starting.