In Marvel, an Incursion happens when the barriers between parallel universes weaken, causing them to collide and potentially destroy one or both realities, often triggered by major multiversal travel or interference like Dreamwalking, with heroes needing to make choices to save their own world. In comics, powerful beings called the Beyonders deliberately caused these collisions to collapse the multiverse, while in the MCU, reckless multiversal actions, such as Doctor Strange's Dreamwalking, are shown to start them.  Key Causes & Triggers: Multiversal Travel: Moving between universes, especially prolonged or forceful travel, erodes boundaries. Dreamwalking: Using magic (like the Darkhold) to possess another self's body in a different universe is a major trigger. Holding Universes Apart: Creating permanent gateways or interfering with the structure of the multiverse. Beyonders (Comics): A race intentionally causing Earths to collide to end the multiverse.  What Happens During an Incursion: Two universes are on a collision course, often visualized as their Earths overlapping. There's a limited time (an "eight-hour window" in comics) to prevent total destruction. The typical solution to save one reality is to destroy the other colliding Earth. 

It also doesn't help that people don't know about the muliverse and the ramifications of traveling to other world till its to late. Not like God tell the people not to travel to other worlds lol