Teach me about bushings vs bearings for the Spur Gear

So, I've had 2 of my builds destroy the spur gear bearing.
It always happens to the bottom bearing.
The shimming is decent and the builds perform great.
But, after a long day on the field, 2 of my builds now have eaten the spur gear bearing. About 3-4k bb's each.
It happened in my UTR45 and I thought I was getting some PME. When I took it apart, I found potential PME signs, but the spur bearing was toasted.

In my most recent build, it happened again. When I took it apart, same thing with the bottom bearing.
I replaced the spur bearings with meteorite bushings and all is well now.
I was surprised the build was still running, considering the bearing was almost completely gone.
The symptoms on both guns were mostly just occasional weak shots and the gun was only mildly more noisy.
I would have expected it to be much noisier and catastrophic with such a failure.

What are the basic physics behind the spur gear being under so much more stress?

Both build are very similar inside.
13:1 short stroked 4 teeth. Piston rack short stroked to match.
Swiss cheesed lonex pistons with POM head, deleted spring bearing
AOE corrected with spacers between piston and head.
M110-M120 springs
11.1v lipo
350fps builds.
UTR45 runs a 22k ASG Infinity
Short build runs Warhead 30K