Question about Lower Unit

[BassFisherman2]

I recently had my lower unit replaced in my 1994 Johnson GT100 by a marine. After having the boat out 5 times, it started to jump. Kinda like you hit something with the front hull. I suspected the shift linkage was out of adjustment. I took it back to the marine and they removed the lower unit and found the clutch dog and forward gear was stripped. They set it back to OBR and they told them the shaft spline was twisted. What exactly would cause the spline to twist other than hitting something because I know that didnt take place? Also what could cause all three to happen at once or could one lead to another?

Thanks
BassFisherman.