Backing up and engine raises out of the water

I have a 2006 Honda 20HP with power tilt/trim

It has been a great engine, but I ran it into some rocks a while back and since then when I throw it in reverse the engine raises out of the water instead of backing up.
If I use the power tilt and raise it up it works great and the tilt motor pulls down once it hits the top. It's the same if I lower it, goes down as it should and motor pulls down once its at the bottom limit.
If I grab it with my hand and try and raise it up it comes up and then returns to the bottom once I let go. What is going on?
How to I fix it so when I back up it stays locked down and pulls my boat backwards?

Pre Thanks for you help resolving this issue!!!

Did you check if your manual trim realease screw ? make sure it is all the way closed turn clockwise by hand gently til its snug. if it feels snug go counter clock wise half a turn then back to clockwise til firm. I have opened the manual release when lower units corrodied out and lost fluid. run engine in revese and stuff 2x2 wood stud in between the bracket to hold engine up which will allow it to be pulled up by hand and locked in full up.

Thanks HonDan!!! Problem Solved.