With modern electronically controlled engines would normally be some electrical hiccup?
Nonetheless, under normal circumstances the aircraft would remain controllable even if both (or indeed one ) engine ran uncommanded to full power. So two probs here, 1- Why did they get (or select themselves) the sudden increase in power on base leg during a DME arc approach. . . and somewhat more worryingly, 2 - why were they subsequently unable to control the aircraft.