Why the storm?

One argument that IT dying caused that, because IT had been such a integral part of the town the process of IT's suffering and death prompted the storm as it was that strong an environmental force.

Another take on it which I subscribe too is the storm destroying the town and killing so many adults was "The Turtle" or some other higher punishing the town. Punishing the town for allowing IT to exist there for hundreds of years and killing so many people, casting its spell of influence on the adults each cycle where they cover up the crimes or deny them. I got the impression that if the town's people had been stronger and resisted more they might have stopped IT generations before. It was implied in the book a few times that IT had influence on people but it was not total, unstoppable mind control. Like when the one guy witnessed young Bev getting beat by her father, he paused and watched for a bit, seeming to debate if he should get involved or not. Then he put his head down, walked in his house and shut the door. The implication is that IT had a role in that but the guy could have intervened if he really wanted to and made the final call on his own not too. I have not see this theory written about too much but that is how I interoperated it.

It was almost ingrained into. Derry and had influence on it's people to an extent. If you recall, when it wakes up it it usually followed by a disaster like and explosion at the ironworks or the storm and flood that occurred when Georgie died. I believe it departure is also followed by a disaster when it goes into hibernation. So because it was supposedly dying, because of the connection to the city it was basically destroyed as well.