Brake fluid boiled and/or the pads were not bedded in properly yet.
I shoul have bled the brakes before this track day, I also should have been more cautious & I probably could have saved it if I didn't panic so much but having 1/10th of the normal braking force at 150kph is scary as hell!
All good in the end though, just a couple of chips of paint missing and no other damage. Got back out there and was a lot more careful, just fanged it down the straight and drove like a grandma everywhere else so the brakes could cool down.