Limited number of possible reasons. Rule them out one by one.
1) D-S overvoltage, including any short spike
- Stray inductance in the system, causing ringing. Ringing caused by stray inductance in input wires can easily go 2x the supply. A lot of inductance in the output (motor, relay) can generate a very high voltage, easily 10x the supply, in which case a properly placed freewheeling diode is needed (to form a tightly laid out half bridge).
- Switching too fast increases the spiking caused by stray inductance. Gate resistor can slow it down.
2) Overcurrent, including any short spike
- Capacitors in the output? They are short circuits and take "infinite" current at turn-on. Look at the pulse safe operating area graphs, taking the capacitor ESR and charging time (depending on amount of C) into account
3) Gate overvoltage
- including ESD shock, especially for small MOSFETs with too little protective gate capacitance
4) Excess thermal dissipation (I^2 * Rds(on))
4b) Excess thermal dissipation in case the device is not conducting as well as you think it is (desaturation):
- your gate voltage is too low, all the time or during some edge case you didn't think of
- you are switching slowly and spending time in high-loss state