No doubt the reason why will emerge before too long. There's all sorts of possible reasons. The A350 is still quite young, could be a gremlin. JFK is quite a busy and highly controlled airport, so presumably it's unlikely that they started off from halfway down the runway.
Nice to see that Airbus's engineers have thought of this kind of thing in the automation, and (SLF) also slightly worrying that they thought it necessary.
It's also slightly vindictive. Suppose a crew have made a configuration mistake. It's better to have that advertised in such an abrupt and public manner by their own aircraft stopping all by itself. But the crew then have to explain it to their passengers and management... The alternative outcome of the mistake is to have their crash or collision damage plastered all over the news headlines.