deskthority

Go from easy steps to harder ones - desoldering components should be one of the last.

First check the resistance from column trace to ground on unplugged board and compare it to other columns. The resistance figures should normally be very high (in MOhm range), however if initial theory is correct ...

The only scenario I can see this happening in is if the column trace in question would have an (intermittent) short to ground somewhere in the circuit.

Check for any blobs of solder anywhere on the PCB, at Teensy pin header connector, and on Teensy board itself.