The RJDC was an organization of regional pilots (mostly Comair and ASA) who believed that mainline pilots were behaving in a "predatory" manner with their scope clauses, because it limited their ability to grow with larger airplanes. Several RJDC supporters filed litigation against ALPA. The main plaintiffs were Captains Dan Ford and Ken Cooksey, which is why you'll sometimes hear ALPA "insiders" refer to the old "Ford-Cooksey litigation." The lawsuit sought to end mainline scope restrictions on regional flying, as well as seeking a large monetary award that would have bankrupted the Association. You can imagine why so many pilots were not happy with the RJDC or the litigation. In the end, the plaintiffs settled their lawsuit with ALPA for a fraction of their legal costs, and the RJDC basically disappeared.
There is a lot of other bad blood between Delta and Comair that goes back a ways, and I won't get into that. Way too much drama.