Mercruiser bravo XR shorts. Trimmed out at top speed and drives trim back in all of the sudden by themselves. First time it was just port drive, second time both drives. It was fast. Like someone hit the switch. Any ideas what it is?

Electrical gremlins would be my guess.

trim lines could be hooked backwards there is a high pressure side to the pump and a low pressure the high has to be on the up side of the trim system.

^^^^ that is correct or too low fluid level or air in the system or bad valve body.

trim cylinders orings. I did 6 this summer on 2 different boats