Clutch sticking in 2006 9-3 2.0t

[el5y]

Hey everyone, my wife was driving home from work on friday and when she down shifted into second gear the clutch pedal got stuck depressed. It stayed like this as she coasted for 10-15 seconds before it suddenly popped back up and has been working fine since. Any ideas on what may have happened??

Thanks for your help

Eric