Lost clutch/ fluid

Trackday today, everything going well then clutch pedal went very light, came back to the pits, parked up.

In the time it took to remove my helmet etc, and go round the car there is a nice big pool of brake/clutch fluid on the garage floor...

Obviously slave cylinder, fluid level in the reservoir had dropped alot, so that is the source.

But why would it go like that, something must have broken such as the slave pipe itself within the bell housing to dump so much fluid...???