Before reading the article I'm going to guess : debris from the impact is now reimpacting back on the main body, transferring back some of the momentum gained on the initial impact.

After reading: okay, this was a Newsround article so its target audience is children, and it's honestly a decent write-up. It did include a similar hypothesis to mine, but said it was further slowing the asteroid after its initial slow down. That's surprising to me because on its face it seems to violate conservation of momentum. So either there's something more complicated going on (orbital mechanics can be counter intuitive), or the reporter got the wrong end of the stick somewhere (orbital mechanics can be counter intuitive)