The ABS system is very sensitive to voltage. If the voltage dips below a certain value (11.5 volts, IIRC), the ABS system faults. It would have been easier for BMW to disregard voltage during starting, as this always places a big strain on batteries (particularly flinging two large lumps of metal in opposite directions).
The reason the Hawker battery never causes ABS faults during starting (and why the gel battery is better than the lead acid battery) is that it has enough amperage to maintain its voltage during starting.