PMOS unexpectedly passing current with capacitor on drain
Is this a simulation?

The leakage current from the MOSFET will charge the capacitor - it is probable that the simulator waits for conditions to stabilize before you get to time zero.

I suggest putting a resistor from the drain to ground. It can be fairly high, eg 1 megohm, to bypass the leakage current. You will also need the resistor to discharge the capacitor and allow the circuit to produce a pulse the second time the switch opens.

Also, many simulators will not function correctly if any nodes do not have a DC path to ground.