If an RS485 wire pair is not driven because all nodes are in "receive" mode then the termination resistors will pull the wires to each other.
In this case noise picked up by long wires can easily be more than the threshold and the RS485 receivers may receive false data.
This is why there is normally a bias network on the wires.
Any RS485 network in which the direction is switched over a single wire pair should have these bias resistors.
RS485 should also have a 3rd GND wire to make sure the common mode voltage does not esceed the limits of -7V to +12V on the signal lines.
"10 ways to bullet proof RS485" is an (old) National Semiconductor AN and well worth reading.
https://duckduckgo.com/html?q=10+ways+to+bullet+proof+RS485Note:
A RS485 driver should be able to get reliable data from a 200mV signal, but it could be triggered by a much lower signal. I would not be surpized if a 50mV noise spike generates a false bit.