Although an orbiting body revolves around a central mass there are other masses that occasionally come close enough to the orbiting body to affect its orbit. This will result in the perturbation of the orbit in a very slight amount. This compounded over time or compounded with other masses could lead to the degradation of an orbit. These objects are somewhat stable in their orbits but these objects are small so basically any mass can change them. If the orbiting body is perturbed in just the right way at just the right time it can travel towards the earth and perhaps even impact the earth.
Meteors are examples of interplanetary objects that actually impact the earth.
One huge factor in our favor is that the earth has an enormous bodyguard out there taking out a great number of interplanetary objects. This bodyguard is Jupiter. Its incredible mass travels along its orbital path sweeping up many objects that would otherwise be potentially hazardous to the earth. Anything that gets near Jupiter gets dragged down until it impacts with its surface.