pmp6712 active buck snubber operation
This is a hysteretic buck converter riding on Vout.  When the voltage at "snub" reaches 75V (riding on Vout so 129V wrt ground) D22 avalanches.  R99 conducts current at that point.  

If the voltage drop across R99 is enough to reach the threshold of the buffer, U15, the buffer turns on the PFET Q27, storing the energy from the spike in L5.  When the voltage drops sufficiently to turn off Q27, D24 turns on (as in a conventional buck converter) and L5 continues to deliver that energy to the output. This cycle repeats for each spike. 
Note that the current in D24 is limited by R99 and R105 in order to scale the voltage across R99 appropriately.  R100 limits current into the buffer input in case of excess input voltage.

The entire circuit is powered from the energy in the spikes, and D21 ensures that the supply voltage to the buffer doesn't exceed the maximum voltage capability of the device.

C81 provides decoupling and energy storage for the circuit.