So the magnet moved on the shaft

[GeeReg]

So I was diving my MT today and I hit the brakes pretty hard. I hear a crunch which usually just means the spur is stripping more than it already is (I have the slipper on order). It's making A LOT of noise though so I look at it and the pinion is almost off of the spur like it had moved. So I bring it in and take the pinion off and notice the set screw is perfectly tight. Then I go to put it back on and notice there's no way the teeth will line up. Hmm the shaft seems to be sticking out awfully far....yeah that's not right.

So I take the motor apart and the damn magnet moved up the shaft so the shaft was sticking out very far from the front and there wasn't enough to slide into the rear bearing. How the hell does this happen? It took a lot of banging to get it back into a normal position, so how does it just move on its own?

It's all back together and working but I haven't tried it in the truck because the spur is so chewed up. I'm assuming the magnet position is one of those critical things, am I right?