Sure it is not some neighborhood kids being stupid and stealing them off the car?
If you hit a big enough pothole or speed bump - it could cause them to pop off. Either tthat or they were not pressed on far enough to begin with.
I've had OEM wheel covers on my various Toyota cars, and only one has ever come off on its own. That was my fault, I ran over a water covered pothole that turned out to be deep enough to bend the steel wheel.
This the OEM wheel cover of aftermarket? I've noticed that some aftermarket ones have some poor springs or none at all - that might be contribuing to them coming off.