Reed's arc is all about him wanting to get into the main event, what better way than to team with Solo's Bloodline? It guarantees him a main event match in War Games at Survivor Series. And from Solo's perspective, he gets another Samoan monster on his side.
Rollin's isn't going to stop fighting Reed because he aligned himself with Solo, so it only makes sense for him to reluctantly join Roman and the OG Bloodline, after all his real issue is only with Roman, he's good with Jey and Sami... probably neutral on Jimmy.
This makes way more sense than Cody and Roman teaming up at Bad Blood IMO