"Backing down the driveway"
Is your driveway sloped?
What is the driveway made of?
Was it wet?
Ase you turning while backing and braking?
First stop...
Road conditions?
Speed?
Pedal pressure?
IMHO if a front brake locks up then there is not enough braking happening in the rest of the system.
The fact that the brakes worked fine after, indicates intermittent or cold (not warmed up) issue.
