When the motor goes in reverse, there is a lock rod that originates in the lower unit that should rise up and turn 2 C shaped clamps on your engine mount. Those clamps grab on a bar that is adjustable in the motor mount, it also determines the angle of the motor on the boat.
If your motor is rising up, either your reverse lock cam was installed improperly and the rod is not rising up at the right time, the reverse lock cam in the lower unit is broken or worn out, the adjustable pin on the motor mount is missing, the reverse lock rod is missing, or the C looking clamps are rusted and don't move.
If you look coming out of the front of the lower unit, you will see the reverse lock shaft, see if it moves. It should only move when going into reverse, the motor is free to rise in FWD or N in case you hit a log. The reverse lock cam can slip over the shift shaft as many ways as the shaft has sides, therefore it is quite ofter installed improperly. This one pictured looks like it only has 4 sides, mine had about 12.
This is a picture of what is down in there, the cam has ramps on it, as it turns, the ramp should be at its highest point when going into reverse.
