Why does gfci breaker trip when something is turned on a separate circuit?

A GFCI breaker trips when a device is turned on a separate circuit because it detects a ground fault or an imbalance in the current flowing through the hot and neutral wires. This can happen if there is a shared neutral between circuits, leading to a difference in current flow that the GFCI interprets as a potential shock hazard. Additionally, if the device being turned on has a fault or if there are issues with wiring, it may also cause the GFCI to trip for safety.