The issue at the time was that they released their results to the press before their results were vetted. They became instantaneous celebrities, and it wasn't clear that it was against their will.
They were very coy about the setup (patent issues? I'm not sure); there was no theoretical model that fit it; and quickly nuclear physicists were pointing out that with the kind of reaction they were talking about they would have been fried by the (radiation) by-products.
The field was very excited (See: "fifth force" for another tale of excitement in my grad-school days). After all, any time there is a result without even a model from which an explanation can possibly derive you are looking at a bunch of new ideas. This means people get to be imaginative, there's sure to be lot's of funding for it, etc. Great times ahead!
Others tried to replicate the results, but since the full experimental setup wasn't being revealed they had to guess. Their attempts at replication failed. This was not the way physics research was conducted, and people started getting pissed off. It was time for more details and proof that they actually got the results they claimed.
With that proof not forthcoming (some started claiming they had built a fuel cell rather than a fusion reactor) they faced a lot of criticism for their PR processes and closed-mouth policy. The damage to their reputation was irreparable.
To my knowledge nobody has yet been able to reproduce the original claims, but I haven't followed in a while.