When tub/shower is on in bathroom #1 a shower other side of house turns on in bathroom #2
Washerless faucets are known to have this problem.
When the pressure drops the weight of the handle can pull the valve on.
Opening the valve and making the springs tighter has worked for some.
Heavier springs are required in the worst cases.
I believe we had 1 person that had it fixed until they opened the garden hose (large flow).
Increasing the pressure may help if you have a pressure reducing valve or well pump increasing the pressure for cut in and off.
But heavier springs or a lighter weight handle may be needed to really fix the problem.