Was the resistance internally changing dropping the voltage?
The battery was either not charging as rapidly as others, or discharging more rapidly than others, or both.
Though rare under normal circumstances, because all bats were same age, and we suspect this battery was subject to physical abuse, I would suspect a shorted cell.
This is very frightening. (Could lead to thermal runaway which can cause a whole lot of hurt.)
My recommendation would be to not turn on any charging source, and get that battery out of the circuit ASAP.
The battery should be inspected for bulging sides.
If so, scrap.
If not, take it to a battery shop, or try to charge on it's own, fully, with a relatively low capacity charger.
If it gases like crazy, at a relatively low charge voltage / current, without accepting charge, this confirms a shorted cell and it's toast.
What would the end result be if I had not fired up the generator and starting electro sailing?
Well, the bank voltage would have continued to decrease.
If the voltage of the remaining bank became too low, some voltage sensitive devices could likely start dropping out.
By turning on the charger, the bank voltage was increased, masking the problem. Again, disconnecting the battery would have been my recommendation.
Would the voltage ultimately drop to zero but, current from the other batteries still be flowing through it?
Could have, leading to thermal runaway, exacerbated by turning the charger on (increasing current flow).