Positive Reinforcement Phenomenon

I have three animals in training that do the same exact thing when they are getting high rates of reinforcement. Two are horses and one is a mule. They are all bay geldings.

Chester, Sparky, and Abandoned Ace all drop their penises and become "erect". They also quit thinking and become quite impossible to train. You can get them out of that by putting some pressure on them to move. A little touch of negative reinforcement and the eyes become unglazed, the equipment gets stowed, and the animal starts thinking again.

Why do these three do this and not the other geldings? The others are all much less "trusting" of humans. They never fully let their guards down (or anything else). Do some mares do the female equivalent? I don't have many mares so I don't know.

The brain seems to quit functioning at these moments. The rate of reinforcement plummets. They seem likely to respond with instincts rather than with cognition so it gets dangerous to be near them.

Is there some neural circuitry that gets activated by the sheer pleasure of earning and eating treats? They don't seem to have that problem if the reinforcer is scratching. It's not a matter of being relaxed, it appears to be true arousal.

Last summer we started withholding reinforcement on Sparky if the equipment wasn't stowed. That helped him a little, but it comes right back when you don't enforce the rule. I could live with things hanging out, but brain disengagement ruins the opportunity to work with the animal at all.