Earth may have had two moons

A Mars-sized object collided with Earth early in its formation, creating two moons: our moon and a smaller Trojan moon. The Trojan moon occupied a LaGrange point, either 60 degrees ahead or behind our moon. Eventually, the Sun's gravity caused the two moons to collide billions of years ago, scattering the remains of the smaller moon across the far side of our moon.

What caused both moons to collide billions of years ago?

What happened to the tiny moon after the collision?