What causes transmission to shift all gears under 20 miles per hour?

What causes transmission to shift all gears under 20 miles per hour?
Your transmission does not know how far you have the accelerator pressed. Some cars have a cable that runs between the throttle and transmission to 'tell' the transmission how far you have the gas pedal pressed. Some cars use electronic sensors to 'tell' the transmission how far you have the pedal pushed. It sounds like your transmission 'thinks' your gas pedal is not pressed at all. This will cause the shift points to occur exactly as you describe them. Look for an unhooked cable between the transmission and throttle.