I don't see why it would start losing value until the -400k loss was reported. That wasn't reported in December before the game launched.... So from that we can tell that EA stock was already falling long before that quarterly report came out.
Im sure that helped things a lot, but the launch numbers were good, they had the largest launch numbers yet. But the stock didnt take off on that news but instead continued to drop. This is bigger than SWTOR. 50% is a huge loss for a company that normally floats with the market.