I suspect it may be the dishwasher soap. If someone doesn't want to put horsey stuff in the dishwasher, they might try just scrubbing the boots with the soap and really hot water.
I'm with you, Mozart. I throw everything in the dishwasher I possibly can. Shoot...I even put brushes in there. DH doesn't like it, but I just say to him, "hey, when you start loading and unloading the damn thing, then you can decide what goes in there."