The Simpsons Movie saw the town of Springfield placed inside a dome due to the mass amount of pollution coming from it, despite the town previously taking an initiative to clean up the lake. However, they learned Homer Simpson was the cause of the pollution spike since he threw a silo of pig excrement into the lake, so the Simpsons were exiled. While Homer seems like the obvious culprit, there are several other suspects who played a part in Springfield being domed, including the following characters.
Bart Simpson
Bart was one of the main causes of the dome-ing, but he was only a factor leading to the misfortunate events that followed. Bart was forced to help his dad reshingle the roof, but he quickly got bored and took advantage of his dad's childlike behavior by suggesting a dare contest. It escalated into the ultimate dare, where Bart was challenged to skate to Krusty Burger and back naked. Bart took the dare, leading Homer to Krusty Burger just in time to save Spider-Pig from being slaughtered; therefore, leading to the eventual disposal of the silo.
Krusty The Clown and His Marketing Team
Homer took a while to get to Krusty Burger and check on Bart, leaving him to get food since they were at the restaurant. Inside, Krusty the Clown was filming a commercial for a new burger and had a live pig sitting on a table for it. After the shoot, Krusty ordered the pig to be slaughtered, but Homer stopped him and adopted the pig. If Krusty and his marketing team had decided against using a live pig or filmed in a different location, their path would not have crossed with Homer's. The father and son would have gone home after their meal and lived their day normally without the pig and without the infamous silo.
Green Day
Green Day played a show at the Duff Summer Concert Series on a barge on Lake Springfield. One would assume that a band as big and environmentally friendly as Green Day would have no problem selling out a stadium anywhere in Springfield, yet they agreed to perform on a barge on a lake that they knew was toxic. On top of that, they recognized the barge was dissolving and waited till the very last minute to make mention of it, causing their untimely deaths. Had they not suffered the wrath of the lake, a certain eight-year-old would not have made it her mission to reduce the pollution in it.
Lisa Simpson
While Lisa was trying to do a good thing for her town, she underestimated the organizational expertise of it. Once she demanded the lake be cleaned up, Mayor Quimby responded by creating an "idiot-proof" barrier around the lake and opening a hazardous waste treatment center to take up all the general pollutants of the lake. Lisa's drive to solve this issue as fast as possible, as well as her new love interest, led to hasty solutions, which led to Homer's own rushed decision to throw the silo in the lake. If Lisa took the time to devise a more realistic plan, there would have been more than one treatment center available to the public, so Homer could've properly gotten rid of the silo.
Lard Lad, The Health Inspector and Lenny
These three participants were more of an indirect cause. Lard Lad's poor sanitation led the health inspector to officially shut them down. In response, Lard Lad had to give away its inventory for free. Lenny, being a good friend, called Homer to share the news, telling him to hurry as supplies were low. The promise of free donuts made Homer rush his one job, haphazardly dumping the silo into the lake instead of waiting to take care of it properly. If the health inspector had come another time, if Lard Lad had been cleaner or if Lenny had called his friend earlier, the town would not have been domed.
The Town Of Springfield
Whenever a tragic event happens, people often look for a scapegoat to blame their problems on. In the case of the dome, the town chose Homer. However, earlier in the film, it was shown that the entire town was polluting the lake. Krusty dumped a tank of flop sweat, Crazy Cat Lady washed her cats in it, Moe threw all his beer bottles in it, and, since the first season, Mr. Burns has been dumping radioactive waste into the lake. The fact that one silo of pig excrement caused a huge enough spike to alert the EPA is hard to believe. While Homer did shoot the gun, the entire town loaded the bullets.
Russ Cargill, The EPA and The President
Finally, it is best to look at the true antagonist of the film, Russ Cargill, who's the head of the EPA. A money and power hungry tycoon, Cargill had a plan to dome a city in America from the start. He even mentioned that he owns the company that makes the dome, giving him a financial motive for the dome-ification. In order to achieve this, he tricked the President of the United States to pick a "random" option out of five. While President Schwarzenegger chose the third option, it's clear Cargill was going to dome a city one way or another.
While it is easy to say the main culprit is Homer since he dumped the silo in the lake, he is not alone in what caused the modification to the town. Springfield had a pollution problem for years. That mixed with the incompetence of the political leaders, the impatience of a young environmentalist and the avoidable death of band all led to the dome dropping on the town.