The puzzling move to release running back Aaron Brown to make room for offensive guard Jacques McClendon has been explained. It turns out that the Lions also released offensive lineman Chris Morris. He was likely the one let go to make room on the roster for McClendon.
This means that Brown was probably released to make room for a new running back. It isn't clear who the new player is right now, but it's very likely Brown was let go in favor of a free agent back.
The reason there was confusion over the earlier moves is because we didn't hear about the Morris news until after Brown was already released. Had the Morris news come out earlier, it would have been clear that he was replaced with McClendon.
Now the wait is on to see who is the other new addition to the Lions. McClendon is one of the new players, but the other one is a mystery for now.