Hydrostatic pressure is pressure that fills voids in a porous material seeping into small openings.
Pumice is a porous volcanice rock that can actually float on water because of all the air pockets.
With less gravity, Pluto's rock hard land ice would naturally be more porous than Earth ice making Pluto's ice more permeable.
These cavities in Pluto's water ice could, under pressure get filled with oily ethane tholin forming hydrostatic pingo craters on Pluto.
We see signs of both pingo processes taking place at Viking Terra.
These tholin compounds are heavy enough to fall from the sky and settle onto the surface.
Ethane’s melting point is 88.6-90.4 K making it the hydrocarbon most capable of dissolving at Pluto temperatures.
Nitrogen's triple point temperature is 63 K and we know N2 reaches triple point conditions. Ethane is an antifreeze agent.