Leaks, malfunctions, etc. can easily be a cause, but...
every time you raise an inflator hose with the exhaust button pressed but there is no air to come out, at least a little water is let into the hose to drain back into the bladder. The more you try to let out air that doesn't exist, the more water will build up. Failure to lift the hose high enough may also allow some water into the hose, which may or may not find its way back into the bladder depending on your next move.
When I see divers struggling with buoyancy and their inflator hose, I know that there is going to be a fair amount of water to drain from the BCD.