2 major reasons mostly.
bad wheel balancing. easy fix. just get the wheels balanced and it ll go away. start from here. after balancing, if it does not go away, get the front brake discs (rotors) taken off and take them to a kharadiya for polishing. sometimes, water is splashed onto them when they are very hot, this causes them to get deshaped and hence vibrations when brake pedal is pressed.