They can either crop up due to some hapless paranormal phenomenon that our weak human minds can barely understand, or due to ridiculous physics.
The Revengerists traverse many dimensions, and very powerful sorcelating characters like Breshvic or Dr. Tasty have been known to leave many Dimension Door Rips in their wake as they move through spacetime without concern for
It is the job of the Great Corrector to fix such holes and maintain order in the cosmos.
Astral Travelers are essentially accomplishing this, but without the use of specific doors or portals, and using only their spirit body and not their physical one.
Those who use technology to travel through these artificial wormholes violently, tearing through the fabric of spacetime, might be comforted to know that on the other end of the door they create, there is always a conveniently situated Earth at the same point in its rotation in space with breathable atmosphere, comfortable temperature and livable gravity. They might be comforted, if that were true. Unfortunately, only a small minority of dimensional travelers (.0000000000000000000000000000000000000000000000000000000000000000001%) are lucky
enough to survive repeated blind use of such a device. The exception to this would be a Stargate, which doesn't send the person to another dimension, per se, but to a distant point in their own dimension, which could also be dangerous if the other end of the Stargate opens into empty space or poisonous gas or the heart of a star or something.
In some places there is a mechanism that binds the universe in such a way that a Dimension Door Rip will open on repeated occasions, such as when a certain number of millenia have passed, when a planetary syzygy occurs, when the ancient incantation is rediscovered and enough blood spilled on the spot, when you put on the holy armor, or whenever a Starbucks is opened up right across the street from another Starbucks.
Sometimes unwitting idiots and losers (Norms) unwittingly pass through a Dimension Door Rip without even noticing all the swirling spirals, whooshing noises, floating clocks, or how incredibly fun the ride is. These unsuspecting fools are dropped into some dimension where everything might be the same except for like one major thing that ends up being ironically tragic. They usually die or get driven mad, which serves them right. The fools.
Dimension Door Rips are not exactly the same as a Nexus of All Realities, which is more like a crossroads or a swamp. These are like nodes that connect many different points, whereas Dimension Door Rips are more like simple hyperlinks.
This allows various stupid dimensions to interact with one another, such as DC vs Marvel.