I have experienced exactly the same thing,except I came off.
The cause was not the engine,but the rear brake locking on.
I was descending a steep hill,in the wet,and had to brake sharply.The rear wheel locked up and despite releasing the brake,stayed locked and I slithered sideways into the edge of the road,did a textbook “highside”and ended up on my arse.
Investigation found that the extrusion under the engine [where the footrest bar is bolted] had cracked at the weld allowing the footrest bar to flex up and down nearly an inch.This combined with me recently adjusting the brake to give about half an inch of travel,and the greasy road,was enough to keep the wheel locked.
Another thing worth checking is that the brake linings are not too worn.I have not had this on a MZ but on another bike,the brake shoes went “over the cam”and locked the wheel.This is caused by the brake shoes moving too far and the servo effect on the leading edge keeps the brake on.
Hope this helps.