MOSFET blowing when soft starting a motor
Given the very low speed at which you are activating the MOSFET, it will spend considerable time in the operating area where it is dropping about 6 volts from drain to source whilst consuming a current that might be as high as 3 amps.
I’m thinking here of the motor being stalled prior to the MOSFET turning on enough to get it actually rotating. It’s not unreasonable to assume that the motor might take 3 amps with 6 volts across it before it starts moving. 6 volts will also be across the MOSFET while in this state and that’s a power dissipation of 18 watts quite possibly.
On the safe operating area graph, this point is indicated by a red circle: -

The graph tells us that if the MOSFET is at 3 amps and 6 volts (half the supply rail) for longer than somewhere between 1 ms and 10 ms, you are going to possibly cause damage to your MOSFET.
Given that the RC time constant is 2 seconds, it's quite likely that you are well into the &quot;unsafe area&quot; for tens if not hundreds of milliseconds.