I don't know, like I said I thought it was a nervous thing. I had a horse that was fine working cattle outside but if we were in the pens or running cattle up an alley he would get a good lather/foam on him and was as nervous as a ***** in church.
Come to find out when the previous owner bought him he got thrown in on a deal buying bulls. The guy threw him in the trailer with 3 bulls, got him home, back the trailer up to the pens, opened the back door. The colt jumped out of the trailer and cleared three fences before stopping.(his wife was a dressage/hunter/jumper trainer and still didn't want to ride him because he was known to buck) Anyhow I think that he was very nervous "penned" up with cattle and that was when he lathered.
Maybe it goes hand in hand, when a horse gets nervous he gets hot...
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.