This is all part of Makima's plan. We've seen that she's able to manipulate even corpses(see her being able to escape hell), and that she is really willing to do anything to achieve her goals. I wouldn't put it past her to have forced Aki and Gun to merge, have a confrontation with Denji, which would cause Denji to *something*(break, evolve, grow angry) and awaken the "Demon the frightens Demons"
Considering what Future said, I'm really worried for Power