It sounds like like you had a sticking caliper on the left (passenger) side. If so, then when rolling down the road, without braking, this would cause drag on the left side, causing the vehicle to veer to the left. At the same time, the brake pads on the left side would become quite hot, and their braking efficiency would decrease. So then when you applied the brakes, the right side brakes would work better than the left side brakes, causing the vehicle to pull to the right.

I agree with the previously stated hint of jacking up the front end, removing the caliper (no need to remove the hydraulic hose), tie the caliper up out of the way, and reinstall the wheel. Then turn the wheel by hand to see if if now turns freely. If it now turns freely, you likely have either a bad caliper or a severely warped brake disc on that side.