Why does my HV monitor circuit (sometimes) fry components?
I found that the issue wasn't related to the circuit, but cabling.  A pulsed high-voltage, high-current line was routed right underneath the PCB on some units, exposing the poor LM339 to extremely high E and B fields.  The units where proper cabling was employed did not have such problems, therefore I'm convinced therein lies the issue.

Future units, however, will employ a lower-ratio voltage divider on the comparator pins to prevent high voltages (>36V, which the pins are rated) appearing there.