Hi,
There are lots of reasons for a vehicle to push (understeer). For the 500/520, its mostly because of the locked rear end. if there was any differential action, it would turn tighter. And it gets worse if you put in a front locker or select 4wd. Once again, this limits the tires from turning different speeds. Wheels that all turn the same speed will push and pull a vehicle forward and resist turning.
But for those of that have added the torque locker, its a trade off in traction is worth it.
In the automotive world, it can be a lot of things, tire pressures, spring rates, weight distribution and transition... it basically comes down to traction in most cases.
Rob