Implying a lot of cynicism on the part of these attorneys.....the search for the CVR had gone on much longer than for the FDR. No doubt the complaint had been in the works for some time; it refers to the preliminary report which has been out for several weeks.
Perhaps they speculated there was some chance the CVR's content could undercut their claims, so better to get their complaint on file, hoping to hedge against the (possibly) unhelpful content. Also as sceh said, being first to file can be beneficial for gaining other clients.