Not much to explain I'm afraid. In the real world accident, the stern went down without so much as a gulp. There were several people who rode the stern and supposedly didn't even so much as get their hair wet. The Chief Baker comes to mind. All I can surmise from this is that the stern was so solidly flooded that by the time it was nearly submerged, there was nowhere left for the water to flow into. With nowhere left to go to, there's no suction to be concerned with.
The air you're referring to was what was trapped in some very well sealed compartments such as refrigerated storage. The sheer weight of the hull section coupled with the weight of the water already in the hull would have been sufficient to drag it down.
Back in 1998, Don Lynch and Rick Archbold appeared at the Seattle Borders and Don was questioned specifically about the "suction" shown in the film. He replied it was (as we knew) a bit of artistic license -- because that's what the audience would have expected and it would have taken too long to explain otherwise.
In the Titanic's case,
a)impact with something that wasn't going anywhere when they tried to leading to
b)excessive weight gain and stresses on a damaged structure to the point where
c)she could no longer remain intact or afloat.
That's the broadly general picture. The specifics get complicated. Is there anything in particular your interested in?