Isolating Power in Communicating 5V Sub-systems
Check the SENSOR_5V rail and +5V rail with an oscilloscope during the power-on sequence.  You probably have a fast edge on the rail, possibly with ringing or undervoltage on the +5V rail during switching. 
The undervoltage on the +5V rail is the most likely and would probably be caused by low ESR power supply filter caps on the sensor module.  You can fix this by putting a resistor in series with the drain of the NFET.  This will form an RC low pass with the gate capacitance of the PFET and slow the switching edges on the power supply output rail which will reduce the peak current requirement for charging the bypass caps on the sensor module.