What causes an asteroid to loose control over it's orbit and enter in to another objects atmosphere?
Asteroids don't have control of their orbit. They are influenced by gravity; they obey the laws of physics.
There aren't any traffic lights for them in space to obey so they know to slow down and stop to avoid a collision with a planet. They'll just carry right on going like someone driving through a red light. If it goes right in front of a planet at the wrong moment then it gets t-boned.
Or it might miss. But here is where the analogy falls apart. Because if someone drives through a red light and misses everyone, they carry on without any subsequent repercussions (except maybe a ticket). An asteroid passing close by a planet will be influenced by gravity between the planet and the asteroid, and it's path will change. That might mean it is flung far away, or it may mean it is captured, putting it on a looping path that eventually draws it in to crash onto the planet.
Asteroids don't have control over their orbit. The orbits are a consequence of the laws of physics.
This way asteroids can just happen to cross the orbit of an other celestial body. Products of such a collision take new orbits, which may or may not result in new collisions.
Orbits, which had been free of intersections with orbits of other bodies, may get moved away from Kepler ellipses over time by the gravitational pull of other celestial objects, e.g. planets like Jupiter or even by stars lightyears away. An orbit can also leave its ellipsis within a short time due to a close encounter with a planet.