A typical failure mode for Molex and similar connectors is some slight imperfection in the connection: bit of grit, corrosion, or sometimes a pin or socket isn't fully locked in the plug block. When pushed together, the pin/socket then shifts backward, so it doesn't fully engage.
Whatever the imperfection, it causes resistive heating or (worse) arcing or both, especially when the pin is carrying a heavy-ish load (amps like the SWG). The result is a death spiral. Arcing makes carbon. Heat causes further corrosion. More heating/arcing follow. Rinse and repeat until everything melts and arcs into black goo.
The defenses are 1) use a good contact cleaner spray before plugging things together, particularly when RE-plugging an old connection and 2) after plugging, verify no pin backed out by inspecting all the stake-on connections at the connector's back. They should all be at the same level.