they were running too fast so they braked before the end of the 1/4.. you can see them braking in almost all of the races.. i guess if they ran too fast then they broke some of the rules like they would need a roll cage or something if they were capable of under 10 seconds which they didnt have.
I do not have too much drag experience however, I have run a couple of tandem events (Autocross/drag combined score).
You do a first run to dial down your car's speed. After your get that time you clock yourself in to set the stage of the race. This is called a Dial in or where drivers must estimate or 'dial in' the time in which they expect to run. Therefore, two unmatched cars in weight and power can compete, by a handicap system. If one runs a faster time than dialed in, it is a breakout and the other car wins by default.