For more background, and since this article is surprisingly short:
This is a better article: https://www.sciencemag.org/news/2017/12/nih-lifts-3-year-ban...
This ban that's being lifted stems from a few incidents, but really the driving force was Ron Fouchier's [0] gain-of-function study where his lab made a few mutations that enabled transmission of the bird flu (H5N1) virus between ferrets. All they had to do was make five mutations and it was suddenly more dangerous (here's the paper [1]). Ferrets may sound random, but they can become infected with human flu virus and show similar pathology to humans - they're a model for humans.
I remember hearing Fouchier speak in Washington at a biodefense conference right after the study came out. He was (is?) pretty much a celebrity among scientists, but the policy folks had a different opinion...
Pretty sure these things will escape from the lab at some point. I've personally handled some B. anthracis (causes anthrax) that was supposedly non-viable only to find out later that it was almost certainly still alive. (Not this case, but here's an example [2] mentioned in the article.) There would (hopefully) be more regulations on BSL3+ organisms like the viruses being discussed, because anthrax is relatively friendly, but like any other field, all it takes is one incompetent person.
[0] https://www.erasmusmc.nl/MScMM/faculty/CVs/fouchier_cv?lang=...
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4810786/
[2] https://www.cdc.gov/anthrax/news-multimedia/lab-incident/ind...