weird limit switch behaviour

Hello!

I'm building a plotter with one of these grbl shields.

All works fine but when I wire up the limit switches they are behaving quite strangely.

When all is connected up they will trigger instantly if close to a powered motor. Things I've tried:

- Testing the continuity of the switches and their cables with a multimeter - The switches do not appear to be conducting when not engaged.

- Disconnecting motors - The switches do not trigger undesirably, but do work as expected if I manually press one

- Disconnecting power to board but connecting motors (keeping usb power) - The switches do not trigger undesirably, but do work as expected if I manually press one

- Removing switch from assembly and powering everything - The switch does not trigger until I move it close to the motors.

So it seems like proximity to powered motors is triggering the switches. Is this a thing?

Thanks!