When you first power up the motors of a 3D printer, they jump to a nearby position that matches the motor current phases provided by the firmware. Such positions occur at multiples of 4 full steps. So each motor will normally jump up yo 2 full steps form its current position. When using multiple Z motors, the problem is that if the machine was powered off with the motors about midway between these positions, the motors may jump in opposite directions.
There are three common ways to avoid this or work around it:
Use a single motor that drives all the Z leadscrews using a belt.
Use the Z probe to level the bed after homing. See https://duet3d.dozuki.com/Wiki/Bed_levelling_using_multiple_independent_Z_motors.
Use a separate endstop switch for each motor.