Could be any number of things. You might have a sticking vent solenoid, a leaking line, a dying compressor- anything.
My vent solenoid wasn't working properly, as a result, it couldn't vent quickly enough. When I hit 60, it would attempt to drop the car, but couldn't do it quickly enough. As a result I would get the CHECK AIR RIDE alert. Sometimes, it seemed, like the vent stayed open, and the car would continue to slow drop while driving down the highway, slamming the rear end.
I just put a new compressor in a few weeks ago from American Air Suspension, and I got a beautiful, silent compressor with a 2 Year Warranty. You can't beat Eddie at American Air when it comes to price, service, or the warranty.