Late bags and an additional passenger boarded after original brake release for pushback results in a breakdown in SOP. Poor communications were cited as contributing.

Narrative

Parked at the gate and ready for departure; 30 minutes late due to late inbound aircraft. Doors were closed; all checklists complete; cabin ready (doors armed); and talking to push back crew; who said they were ready for departure. Called for push and got clearance to release brakes from push crew. Received push clearance; and were then informed by push crew they had 5 more bags. Contacted ramp; they canceled push clearance; and I informed the push crew they could load the bags. About this time the jetbridge began coming back to the aircraft. I informed the purser; and reset the parking brake. There was NO warning the jetbridge was coming back; and as the flight attendants were already well into their demo; (not aware the jetbridge was coming back); this had potential for disaster with a slide deployment. Due to the cycling of the brakes; the ACARS showed return to blocks; and had to be reinitialized; the flight attendants had to repeat the demo in its entirety as another passenger was boarded; and we re-accomplished the pushback checklist and confirmed once again the ground was ready; all bags were done; and we could again call for push clearance. This was a great effort on the part of the agent to get this passenger on the flight; but based on the timing of events to this point it had the potential for disaster. In the past; management has been reluctant to make such an effort for late passengers in a situation like this. Perhaps that was for a good reason; and this is a good example why. In the future; there needs to be BETTER COMMUNICATION in situations like this; especially with the Captain; and it should not be left to chance that the crew sees the jetbridge returning. For whatever it's worth; the push crew either didn't notice the jetbridge returning; or they did and said nothing.