The ET pilots were trained with the MCAS system. But to speculate: ET pilots seem to have experience issues before the MCAS would be active, so it's not unlike

The ET pilots were trained with the MCAS system. But to speculate: ET pilots seem to have experience issues before the MCAS would be active, so it's not unlikely that there was another issue with the plane, compounded by the MCAS kicking in when pilots where already fighting the aircraft...