Capacitive noise
When an AC cable is lying close to the signal cable, there is a capacitive link formed between the two cables. This link, sometimes called crosstalk or pickup, enables a small current to flow from the AC cable to the signal cable. When the signal cable is in the off state, this small current may affect the I/O relay concerned.
For instance, this may cause the electromechanical relay or optocouler to "latch on", LED can glow when the signal is off or in worst case it can switch the relay on even when the signal is off.