I like this youtuber’s explanation.

m.youtube.com/watch?v=1eM9z18NezI

I think they are different between the mcu and the source material comics, as is usually the case with marvel (Ultron, Thanos and the infinity stones work differently too)

In the MCU, in Dr Strange 2, they insinuated it happened simply because of abusing multiversal travel. In spiderman 3, it happens/almost happens because of Strange and spiderman abusing magic/messing up q spell (i think this is stupid because why should one spell reach beyond realities or have that power? Maybe it shows that universes were already colliding or approaching. But i digress...)

In the comics it seems that
- the multiverse is a set of universes each in their own bubble, existing in a beyond-3D space. E.g the fourth or fifth dimension is the space between different universes
- incursions are actual collisions of one universe against another
- Beings known as the beyonders wanted to destroy the multiverse for some reason (presumably they created it? Hence wanted to clean slate again?) 
- Eventually universes start to collide, which will lead to multiversal collapse
- For plot convenience, they collide earth-to-earth, which is possible because the multiverse exists in a higher dimension than a universe does, so can collide at any point without the edge of a universe having to meet another edge first.

Sources: [Superhero toy store - marfel incursions](https://www.superherotoystore.com/blogs/news/marvel-incursions-explained-how-the-multiverse-collapsed-before-secret-wars?srsltid=AfmBOorysKTm9GFY8aWD9QoQ693sa9NsfT2WL5Z5EAVX59hNkkIJxfLi) (TW: Gambling advert(?) built-into site)