Strange Clutch Behaviour

Hi all, something a bit odd has happened with my Mk2 VRS CR TDI. My girlfriend was driving it on the motorway (I wasn't in the car) at around 60 just slowing down as traffic built up, when the clutch seems to have overheated and started slipping. Apparently there was the usual clutch burning smell and loss of drive. The AA man confirmed it was probably this.

This was yesterday and it's ok now. It seems very strange that this happened while driving along the motorway. She's done it before riding the clutch up a massively steep hill in Sheffield in heavy traffic, but learnt her lesson! I also trust her account of what happened.

Does anyone have any idea what could have caused this?

Another point to add, maybe I imagined it, but I'm sure I've felt a bit of light juddering when pulling off in first lately. It's only done 60k gentle miles though.

Thanks!