I put another video up of the disassembled hydro, better this time I hope.
https://youtu.be/SVzFiva78mE
Could it possibly be that both relief valves have failed open and at some point after some seconds of fluid flow slam shut and suddenly cause the spinning pump's flow to be diverted to the motor? I bought both valves a year ago from MachTech. (Actually bought 4, put the other two in the front axle.
I did put a pair of used manual valves in and got the same result, so that is why I've not gone down that path.
I am trying to find a replacement locally, but the one source I've used before had just sold his entire stash of Cub stuff. He's going to try to see if the buyer will let me buy one of the hydro rear ends. Otherwise, I'll keep looking locally, call Scott Urschel or Ray Weaver.
I'd still like to understand the cause of the problem, so as you've suggested, I'll remove the piston block springs and the swash plate. I have most of the parts from another hydro in my spares inventory if I need something.