So today I've had at least 6 packed tiles across several pens of horses, where normally I don't see any packing at all since I put in troughs. One or two would be noticable, but six is a lot! It might be related to a movement bug reported in cahelp by Crimsonearth? I did have one of my horses follow me across some packed tiles as I was replanting them.
EDIT On further inspection I have at least 10 packed tiles across several pens, and my champ rat is pacing back and forth across one tile obsessively. She either needs a dance partner or a tranquiliser.