I may be wrong but I seem to remember reading about a sensor on the car that assists braking when you have four or more occupants in the car at once and there for needing less pressure on the brake pedal.This sensor may be sticking .
Yep, some cars have a hydraulic valve/regulator that regulates rear braking force depending on how much weight there is in the back of the car from passengers and luggage. If the rear brakes are applied too hard the rear wheels can lock up as weight goes forward on braking and that can cause a skid/crash.
Correct, but NOT on a car with ABS fitted
Hence the word SOME