Here is a wild theory for those who really understand these things to pull apart.
EHU plugged in and runs out of credit - Multiplus switches to inverter mode - Inverter provides different AC using out of phase 115V down positive and negative wires - EHU still plugged into bollard where negative wire is earth bonded - Inverter sending 115V to earth - 12V battery powering drain to earth.
Answer, unplug the EHU as soon as credit runs out?
EHU plugged in and runs out of credit - Multiplus switches to inverter mode - Inverter provides different AC using out of phase 115V down positive and negative wires - EHU still plugged into bollard where negative wire is earth bonded - Inverter sending 115V to earth - 12V battery powering drain to earth.
Answer, unplug the EHU as soon as credit runs out?