Both the pseudocrater and pingo explanations would require liquid water at this extreme southern polar location, which presents a puzzle.
This animation shows the temperature might reach at best -40 C during late Spring at 87S latitude:

Perhaps during Summer it could get a little warmer, maybe to -30C. Still a puzzle how you would get liquid water.
It could be volcanic. But the appearance of the feature suggests that it mirrors the seasonal melting of the CO2 cover in the area, which implies solar heating in its formation.
One possibility is that it could be due to CO2-water clathrates which are expected to form on Mars:

PTX PHASE EQUILIBRIA IN THE H2O-CO2-SALT SYSTEM AT MARS NEAR-SURFACE CONDITIONS.
http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1689.pdf

The report states the transition to liquid water brines would occur at 262K if they contain NaCl salt, but this could be down to 230K if they contain
calcium chloride salts.

Bob Clark