"What is a sulfated battery? Batteries generate lead sulfate during discharge. Battery sulfation occurs when lead sulfate present on the battery plates after discharge is not converted back to usable lead during charging service."
While charging is usually controlled to avoid overheating...not so when discharging, repeated heavy discharge could have boiled your electrolyte and flushed the sulfates on the plates out onto the top of the battery. As battery voltage is drained, amperage increases corresponding. Some battery charging systems have an equalization charge which is somewhat like an overcharge to get things stirred up.