Might not be the radio itself, but one of the steering wheel switches or the wiring between the switches and the radio. If a switch is worn internally or the wiring is nicked, a bump could cause unintended contact.
Here's what would make that more likely than the radio being the culprit: when the station changes, does it go to another preset station? If it does, and the buttons on the wheel can only choose among presets, then look at the switches/wiring. If the station changes to another one that isn't preset, then I'd suspect the radio.