Nexus points are the whole reason the world is in the state that it's in.
They exist in every dimension, across time and space, fixed points that link dimensions together. In the simplest terms, nexus points are where the walls between dimensions are thinnest and weakest. They are leaks in the hull, where magical energy seeps through from one world to another. They are epicenters of strange phenomena, unexplained disappearances, sightings of mythical creatures. In short, they are weird central.
When apocalyptic nuclear hellfire rained down on the world, the nukes were specifically targeted at various nexus points around the world. The energy released by the simultaneous nuclear explosions upon the nexus points caused what little remained of the dimensional barriers to simply tear open.
The end result was a flood of magical energy from many different dimensions through these nexus points and into the world, changing it forever.
Over time, the dimensional barriers re-formed, but the abundant magical energy surrounding them means these locations are still unstable and unpredictable.
Many people have attempted to use the nexus points to create portals to other dimensions. The Fae of New Tairngire, for instance, have spent decades attempting to sabilize the Stonehenge nexus point into a reliable portal back to their home dimension. However, this is no easy feat, and is in fact incredibly dangerous.
It is impossible to know where a nexus point will take you. The other side is continually shifting between an infinite number of dimensions. Likewise, it is impossible to guarantee your safe return back to your home dimension. The portals, once opened, are incredibly unstable and may collapse at any moment, leaving you trapped in a strange dimension - as has happened to the numerous 'Travellers' in this world.
Similarly, you opening a portal to another dimension means that whatever exists on the other side can cross over here, and may not be nearly as nice as you. Therefore, most of the major governments and corporations of the world contribute to the monitoring of the nexus points to ensure there is no rogue group attempting to open up a portal that may potentially doom us all.