I just did a bunch of reading through Google on this topic in general. Most of the problems that I came across were older vehicles, but one was as new as 2007 - it pointed to a bad release bearing.
Most of the issues pointed to the MC though, on the other results. I'm curious... does this happen with the car in the OFF position as well?
Clearly - one thing is certain. Your pedal is either A) sticking at the pivot point, randomly... or
not receiving enough pressure back from the system to return itself. If it's the pivot point, maybe something is broken/breaking or loose. If it's the pressure, it could be anything from the clutch fork not returning all the way back to the MC not getting return pressure.
When this happens, does the clutch stay disengaged? When you pull the pedal back up, does the clutch return? Is it happening when you dry-pump the pedal, with the car off?
Just some questions & more than likely obvious suggestions...