I'm thinking something like half the tread hit slippery surface, and the other half didn't, torqueing the rear wheel in one direction and throwing it to the side. The throttle to maintain speed as you say must have been enough to throw it. Happened very quickly, and as soon as the slick patch went away and the wheel gained purchase I was thrown off the bike as obviously there was misalignment between front and back wheel and the momentum of forward motion.
I don't think something like that can happen on an typical uniform slippery surface. I rode motorcycles exclusively for some time in seattle (rain capital of the US) and never had that happen. Rode motorcycles year round off and on for ~10 years before this happened, it was a freak occurrence. I think it requires a particularly nasty freak occurrence of the shape of a wet patch and the angle and entry point at which you hit it.
Unless you have a rear tyre profile like that of a regular car tyre this is very unlikely. (People who ride long distances on the highway do use car tyres) The contact patch width is TINY.
Well it took me a good 10 years with most of that exclusively on motorcycle before I crashed like that in the rain. I do agree it's unlikely, almost a freak accident to have that happen while going in a straight line at constant speed on good terrain. I truly have no good explanation.
I am no expert but if the whole contact patch hit something slick (oil patch from someone leaking in stop and go traffic) the tyre would first lose grip then immediately gain traction. This is like the definition of a high side anyway. Though I am not sure how rain effects things.
I don't think something like that can happen on an typical uniform slippery surface. I rode motorcycles exclusively for some time in seattle (rain capital of the US) and never had that happen. Rode motorcycles year round off and on for ~10 years before this happened, it was a freak occurrence. I think it requires a particularly nasty freak occurrence of the shape of a wet patch and the angle and entry point at which you hit it.