This also happens when you kill a non-combative creature like rabbits and deer. The game believes you're still in combat for some reason and you can't mount or interact with things for a few seconds.
One of the causes is the netch pet. Seems like sometimes the game thinks u killed a good-old-days-critter when it despawns.
Do a Dodge Roll and it clears this condition. I think mashing the Escape Key to get back to a Menu a few times can do this too, but the Dodge Roll is more reliable.