What could have caused the MOSFET to fail in this high-side switching circuit?
M1 could blow up if the Gate Source voltage is too much. Autoelectric systems are full of spikes. Place a zener between gate source of M1 to clamp the gate below the gate source rating. For example if the Vgs rating is 20VDC then a 18Volt zener would suffice. Load inductance is always present. This causes drain voltage spikes at turnoff which could exceed the avalanche rating of M1. Use a freewheel diode or snubber or clamp to adress this. If your load had a short M1 would blow up. The high prospective fault currents would exceed the current rating of M1. Normal fuses are too slow to protect mosfets. Consider some form of current limit to protect M1.