It appears that NF₃ absorbs strongly at 900 nm. This is a wavelength NOT absorbed by CO₂ or O₂ or O₃. So radiation that would otherwise be emitted to space at around 900 nm (near infra red) will be absorbed if NF₃ is present. It appears that the lifetime of NF₃ is about 700 years. So, yes I think the reason it is a stronger greenhouse gas is based on the different vibrational modes for NF₃ relative to other greenhouse gasses, coupled with the fact that it degrades slowly. Hope this helps, write back if you need more info, I will do my best!