"Nobody Knows" would be the QI answer..
The clutch bearing has probably come loose in its locating slot and is now whirring around, which is what makes the noise. That places extra wear on the outer "fingers" of the clutch until they are worn through and then the bearing "falls" into the crevice it's dug for itself, so you lose activation of the clutch.
It doesn't hurt the gearbox and the bits that are eating each other (bearing and clutch fingers) are all parts of the new clutch kit, so there's no danger of breaking something you're not going to replace in a minute anyway.
You can give your clutch an easier time of it by sticking the car in neutral every time you come to a stop, rather than holding the clutch in, with the car in gear (if that's your particular bad habit). However when the beast goes it'll go without warning so I'd make sure you have the RAC number handy..
When you change the clutch, I'd stick a new slave on there too unless yours is remotely new. Job done "forever", bearing in mind the car's reasonably expected lifespan..