First, this does not happen for all nzbs, only some. I haven't noticed any similarities between the ones where it does happen.
I use SickRage to grab nzbs. This has been happening for a few months now.
These hanging downloads will also persist through restarts of sabnzbd. The only fix I have is to kill the download and re-add it. Oftentimes, the same nzb, once re-added, will download just fine.
Here's a screenshot:

Technical details:
Sabnzbd version: 1.1.0 [8abcf08] installed on Ubuntu 14.04 x64 via JCFP's PPA.
My sabnzbd.ini(ALL CAPS = edited): http://pastebin.com/A9udL1Xc
Any thoughts would be appreciated.