The explanation was supposed to be that the RV was trailing behind, saw the flare, and assumed that it was Glenn's car so went ahead to reach it. Rick's group saw the flare too so they ran through the forest to get there.
Due to poor editing though, they showed the headlights of the RV in Glenn's car moments before he plowed into zombies, meaning they shouldn't have been that far apart. One freaking line of dialogue ("we'll go first, if we run into trouble, we'll shoot the flare") would have fixed that entire mess.