As said the clutch pedal mount is a Mk4 problem, not a Mk5.
Going to likely air in the system, I would suggest as above - inspecting the hydraulic circuit, if no leaks are found I would suggest bleeding through new fluid and ensuring there is no air in the system.
It could be there was a small bubble which at that specific time caused a problem.
Hear it happening loads when mates change out slave cylinders, bleed it through, works all good, drive down the road and end up with clutch pedal on the floor, a quick re-bleed sorts it.