We can only speculate, but Audi had similar problem in late 80's or early 90's, with run away cars. When they investigated, most cases happened on the parking lot and drivers were older, and at that time there was no shifter lock for park position, you could shift from park without pressing brake pedal.
In all cases drivers swore they were pressing the brake pedal as hard as they could, but the car kept accelerating, it turned out they were pressing the gas pedal, so in panic situations weird things can happen.
It may not be the case with Lexus, as back then there was no electronics with wierd logics to interfere with driver's input.