Thank-you both for your replies.

It's a Heng Long 1:16 M4A3 Sherman 3898 with metal tracks. Warnings are fine, but it's nice when you know the "why". I imagine the treads having wider spacing as they go around the front and rear wheels could pick up something solid, and then when the treads close up, the grit keeps them forces apart and they break apart. Wear from grit is more easily understood and avoided.