Any reason why a serial connection might interfere with a setWatch for a contact on other ports?

I have a phone module connected.

I also have a setWatch on a port where i look for a loss of contact between D27 and D28:

digitalWrite(D27,1);
pinMode(D28, "input_pulldown");

setWatch(function() {
  sendAlert();
}, D28, { repeat: true, edge: 'falling', debounce: 50 });

The serial connection over D29, D30 and D31 seems to set off the contact watch. There is no soldering on the board that could be causing a short, just an unpowered serial device