What causes premonitions?

Answer 1

Premonitions can have a variety of sources and some are more mundane than others.

Sometimes premonitions are simply the result of chance. We imagine that something is going to happen, and it does (with the billions of people on the planet, there is a 100% likelihood that SOMEONE has had this happen, though the chance for a single individual is significantly less, though an actual percentage is nearly impossible to determine).

Sometimes, the human mind experiences a sense of "deja vu" which is a result of a "hiccup" in the temporal lobe of the brain (generally coupled with memory) which gives the impression that one has experienced the situation before and then anticipates what is about to happen based off the earlier, similar experience.

Sometimes, premonitions are the result of subtle cues we pick up without being directly aware of it: Things like people's body language or atmospheric changes can trigger subconscious reactions perceived as premonitions.

Then there are more fringe explainations:

If time is linear and, more or less, deterministic, it may be possible to access the already determined future. Of course, if you accept this idea, the idea of free-will disappears completely.

If time is relativistic and therefore able to be deformed (Einstein's space-time), it may be possible to likewise able to access the "future" just as one is able to go up and down stairs. Of course, this too eliminates free will.

The problem with premonitions is that, if they "really" exist as in the later examples, they mean that the future is unchangeable and therefore we have no free will and are merely "newtonian/quantum machines". Some people are fine with this idea, others, not so much.