From what I've read, that's caused by Beejive crashing in the background and Backgrounder not knowing the original instance is no longer running. If you background Beejive it works fine, but if you open Safari or something and browse a bit, the amount of memory it takes up with cause Beejive to crash. Someone was working on a script that would automatically check for crashed backgrounded apps and restart them, but nothing ever came out of that I don't think.
If I background Beejive and don't use any other apps at all, it will work for a very long time. If I background it and then use Mail, Safari, Facebook, it no longer works in the background unless I reboot the phone. It's a memory issue (part of the reason Apple doesn't want background processes).