Sounds like it's probably developmental, but it might have turned into a habit since it's at the same time. Have you heard of wake-to-sleep?
I would suggest a couple of things. First of all, don't rush in unless he is crying. If he is just talking or fussing, try to let him settle himself. By rushing in, you could be enforcing the waking.
Second, have you tried swaddling? It was a lifesaver for us at this age.