Sloth is the most likely explanation, but a legitimate distraction may also play a role.
I live within scanner range of the 503 detector (Esperance Station, at the east end of the Delanson passing siding). Some of the crews deliberately mumble, and some don't acknowlege at all.
To be fair, there's always the possibility that something was going on in the cab at the time, and that took priority over talking back to the detector. If a unit stopped loading, or they just dumped the air unexpectedly, I could imagine that would be more important.