I really don't understand it either. It can't be heating
or current, because the resistance of the lead is a few orders of magnitude less than the resistor itself, and the resistor does not look like it was ever really hot. And it doesn't look to me like the lead was overstressed by the installation
To me it looks like errant cutters, too. Maybe a rushed tech used cutters to grasp the lead to bend it over, and that created a weak spot that became a failure from vibration?
I would just replace the resistor, or (more likely cos I'm cheap) use a short piece of wire to replace the broken lead. The short piece could be a different gauge, or a piece of stranded wire even, if vibration is the culprit. Tape a couple spare resistors inside the case, and carry on.
(btw it looks like an 18k resistor - brown-gray-orange. 16 isn't a std value)