There are two families of asteroids in the asteroid belt, Alinda and Griqua, which have quite eccentric orbits, and their orbits become more and more eccentric as time passes. From time to time they come close to Mars (or other interior planet), which slings them in a random direction. Some may end up hitting us. That's how the Kirkwood gaps in the asteroid belt are formed.

https://en.m.wikipedia.org/wiki/Kirkwood_gap