I had a similar problem 2 weeks ago. someone was talking to me when I was
doing my final checks and I did not check that my brakes were closed and
locked. As soon as my Discus was airborne behind the tug the brakes popped
open which dropped me two feet on to the ground. The brakes then slammed
shut so I took off again - so they opened again! Repeat.
This whole sequence took about a second. I had my hand on the release
knob so I pulled off and then grabbed the airbrake lever that was flying
backwards and forwards bruising my hand in the process, and landed
normally straight ahead.
Lessons learned.
NEVER let your self be distracted on checks.
Have your hand on the release. There is not time to be groping for it when
things go wrong at low level.
The next flight in a mosquito on the winch I accidentally knocked the
flap lever into full negative with my knee, but that is another story.

Nigel