How does this shunt regulator work?
The detail you might be missing is that the ADCPIN is not always high impedance, because this device has internal cell balancing.  The external P-MOSFETs are for external balancing, which allows for faster balancing and better input filter for the ADC.

When the internal balancing is turned on, then adjacent ADC inputs are pulled together.  For illustrative purposes, imagine a low-value resistor suddenly connected across C1 in your circuit.  This basically pulls down Q1A's gate, turning it on.

Here's an example that although uses a different chip and N-MOSFETs instead of P-MOSFETs, might help explain how it works (in general terms):



I also found a related P-MOSFET example provided by TI in this forum: