They are sand bags (or have some other material in the bags) for weight and balance, and their use, especially in CRJs, is more common than you might think. I once was in a CRJ at DTW that taxied out to the runway and then had to turn around and come back to the gate to take on about 3 or 4 bags. The numbers weren't all calculated when we left the gate, and it was later determined the aircraft needed the bags to stay within limits.

I've seen them loaded in ATL as well. I wonder how they account for all the sand bags in the system when they get offloaded at outstations.