problem in PCB.

I can connect loads to my PCB and transfer them the AC mains voltage and current.
Also I have a switch and a led, so everytime i press the switch, the led toggles.

I noticed that when i connect a load to my PCB and use the load's own switch to switch it on and off, the led toggles, even that i dont press the PCB switch.
After i disabled (in MCU) the option that pressing the PCB switch will toggle the led, then the LOAD didnt toggle the led anymore.

What do you think is the problem?
I dont understand how switching the external load, simulates pressing the PCB switch.

thanks.

Please show us your schematic diagram.