You are so lucky! Mine has no problems dropping during every grooming or immediately after every ride, but God forbid I should touch it or clean it. It’s the only thing besides flies which makes this horse grumpy
I haven’t seen it mentioned yet but have you tried Vicks Vaporub smeared in his nostrils ( not on his junk )?
It is a temporary aid working with breeding stallions.
I’ve had geldings do this as described by the OP. I note that in a herd, mares that are not receptive to breeding, can be pretty rough on an interested stallion. They may snarl, bite, and/or kick the offending Romeo. I don’t suggest biting or kicking, but perhaps a sharp correction and a good “trainer voice” snarl will remind him who he is dealing with, regardless of what set him off.
People seem to think that geldings exhibiting this behavior were “gelded late” or “proud cut.” That almost never happens. I’m working with a gelding now (age in his low 20’s) that the owner insists was gelded as a 4 year old. I looked into his background (he came from a very well respected breeding farm) and found that is patently false. Yet, he can still mount and penetrate a mare. And he’s not the only “normal” gelding I’ve seen do that. Horses are individuals and sometimes geldings get randy.
This is going to sound insane BUT my friend has a mare that acts like this (minus one detail) when she is on grain because it flares her ulcers. No grain, all forage and the ulcers/discomfort/stress abate, along with the ‘studdy’ behavior. I’m not talking about a one off, this has been a consistent pattern over the years and I’ve witnessed it in person.
So, its possible this is a stress/pain reaction too which also pairs with the scenarios you were describing. Obviously this is anecdote, not data. But horses can make weird connections depending on the timing of scenarios and behavior, which is also why ‘dropping’ and +R training are a thing.