Giving new meaning to "Bow-chicka-wow-wow"

All of my dogs and cats have been neutered or spayed on the very day they turn five months old because my vet won't do it before five months. (Or when they are older animals: before they enter my house.) Because of this, none of my animals have ever had problems with marking or humping. One of my hounds--a girl named Zelda--occasionally mounted my old yaller dog Zeke, but that was totally a dominance thing; she thought she could steal "alpha dog" from him, alas, to no avail.
Then there was Zev, my "special dog." (See previous post.)
He will, once in a great while, try to hump his older brother, Hahn. I do not think it is a dominance thing. He seems to genuinely enjoy it, but we actively discourage it and it has been quite some time since the last "incident."
Until yesterday.
I took both dogs to be groomed. It was Zev's first grooming. When we came home, until bedtime last night, he repeatedly mounted Hahn every time he thought we weren't paying attention.
Poor Hahn.
I have two working theories:

1. He felt so sexy from having been prettied up that he just had to work it.
2. He thought Hahn looked pretty damn fine after a day at the spa.

Either way, he seems to be "hump-free" today. Thank god.