Originally Posted by
Digital_Cowboy
It was the QR having come loose. I tightened it up before my ride and the problem was solved. Thanks to everyone for your help.
So the question is: do you have a good quality enclosed cam skewer? They're a must for horizontal rear dropouts and front disc brakes. The inferior exposed cam skewers will slip. If you have one, I strongly suggest you replace it. You don't want your front wheel coming out during hard braking!