I looked at a number of the failed transactions on skanner before the api facility was stopped and it seems a lot of withdrawal transactions had several inputs, but one of which had zero balance, an initial double-spend. For example. If customers A and B both try to withdraw BTC within a few seconds of each other their payments would share one input address in common with a balance to satisfy only one customer. Whichever gets propagated through the network first has a successful withdrawal, the other is silently ignored, languishing.