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.
