I doubt that it's a case of your radio switching to an ATC frequency, as much as it is a case of ATC transmissions being strong enough to be heard through that radio. A very strong radio signal can sometimes be heard on a radio that isn't tuned to that particular frequency. I live directly beneath a common departure route from PIT, and on occasion I'll hear ATC transmissions over my kitchen radio as well. Someone with a better grasp of the technical aspects can go into greater depth, I'm sure