I'm confused by your issue, but it's late and maybe I'm missing something.

There are track pins which are the metal rods that are holding the track links together.
Then there are the guide horns on the track links themselves that keep it centered, which can slip past the idler (rear) wheel and cause the track to come off.

Are the pins backing out of their slots and sticking out or is the track as a whole slipping off the tank?

Welcome, by the way.