Probably simultaneous application of the brake pedal and throttle pedal.
While the brakes can hold back a full-throttled engine when the two pedals
are independently depressed, with sufficient force to the brake, this does
not necessarily happen when the single braking foot contacts both pedal
You can experiment with this phenomena on a deserted road or large, empty
parking lot. If your braking foot is placed off-center to the right,
overhanging on the brake pedal pad, and depressed, it will contact the left
edge of the gas pedal and the truck will lurch forward. The natural
reaction is to press harder on the "brake" but the throttle will overpower
the brake because maximum engine torque will be reached before maxium brake
action is achieved.
The geometry of the brake and gas pedals in the Ford F150 series is
completely wrong. Just look at some 50's cars. Their brake pedals had a
long travel to work non-boosted master cylinders. So the pedal was about a
foot off the floorboard. The gas pedal was way down low. There is no
physical way for both pedals to be contacted simultaneously. As car seats
were lowered and boosters became common, the brake pedal travel was reduced
and lowered, creating the hazard of sudden acceleration. One final note is
gas pedals are ratioed to work with little physical movement to get the
throttle to go wide. Check this out yourself. Depress the gas pedal about
1/4 inch and your truck will get up to about 60 or 70 mph. With this much
power on hand for so little gas pedal depression, it is no wonder that
accidental simultaneous pedal application causes accidents, not limited to
old ladies. It could happen to any driver, no matter how experienced.
Just a new pair of shoes is all it might take to confuse intended pedal
Definitely a cause for legal action.