Because with the front transmission belt pinned to the rear trans, you can now use all the power you have in the truck. That puts extra load on the idler gear, particularly the rear one. And we were already driving these trucks before this started, the gears were likely not perfect so add grip and add power and ker-blammo.

When the decoupled slipper was working, it would torque bias to the front or rear, meaning it would slip all the time, and the transmission took less load. The solution is to replace the gears on a regular basis and to set the slipper looser (similar to how an SC10 would be adjusted)

I could not believe how much more throttle I could use when I drove mine. Previously, I had to drive the truck really conservatively else the chassis would do bad things. Now I want more motor!