MCU pull down input push button with interrupt ESD issue
I think #2 is quite plausible, especially since you mentioned plastic case.  You probably generated some static electricity while handling the plastic case, and that got discharged thru the switch line.  The switch is probably sticking out more than other things, so would be more likely to get zapped.

If this only happens when you first install the board in the case, then you can just ignore if necessary.  However, something is marginal, and I'd want to make the system more robust.  There are various options:

Make the pulldown stiffer.  If this unit is not battery powered, then changing the 10 k&Omega; to 1 k&Omega; is a easy thing to do.  That will use 25 mW when the switch is pressed.  If this is line powered, than that's irrelevant.

Put a capacitor to ground on the switch line.  This will require more charge on the line before it gets to a logic high level.  A 1 uF ceramic is cheap and small.  Even with the 10 k&Omega; pulldown, that's still only 10 ms time constant, so is irrelevant in human time.

Put a R-C low pass filter after the switch and pulldown.  For example, another 10 k&Omega; in series followed by 1 &micro;F to ground.  The extra delay is still irrelevant in human time since you have 10 ms time constant towards the switch on state and 20 ms towards the switch off state.  This will definitely filter out short spikes, and provides additional impedance for the pin protection circuitry to work against.