what is wrong when the shift locks when changing from park to reverse?

Your shift interlock system is malfunctioning. There is a solenoid mounted underneath the shifter that releases the shift mechanism when it receives a signal from the brake pedal light switch. It is somewhat common for the solenoid to become dirty from spilled drinks, dust, etc and intermittently work. Next time this happens, have someone check to see if your brake lights are working, if they don't illuminate replace brake lamp switch.