The NRA was working the
Seegars case at the time, a similar case in the same jurisdiction. That case was running into trouble, and
Parker was doing well. In an odd combination of wanting to join the winning case, and being scared that the better case would implode and cause huge problems (i.e.: SCOTUS rules against individual RKBA), they simultaniously wanted to join the cases to improve their own case's chances, and to derail the case if it started heading in the wrong direction.
The guy behind Parker knew he had a good case, and was funding & running it himself, and didn't want anyone else screwing it up.
In the end, Seegars failed, and Parker very nearly did (only plaintiff Heller maintained standing thru the latest verdict).