Faulty supply cable comes to mind as another simple cause. If for some obscure reason the contacts of plug have developed high resistance connection with wire, then this could happen.
To check for it, using ohm meter, check the resistance between the pin on the plug and it's corresponding wire connected to switch.
I presume the plug has three pins viz. Phase, neutral and earth. If the wire for phase or neutral connection has high resistance, you may have the switch on problem.
Warm Regards,
