Don't ask! Watch it, it's part of the plot.
If you must know
Nothing is known, not even by the scientific team that went into the zone, but an analogy to a prism is made, the zone is a prism that refracts everything, including DNA, like a prism refracts light.
So this bear killed a girl, and now the bear is either mimicking her screams as a mutation/evolution to attract prey, or even scarier, the zone merged a piece of the girls consciousness into the bear, so the girl is actually living in that piece of her consciousness,which is actually the moment of her death, in a loop, as the bear tries to attract prey.
Nah you do, Caped Baldy, don't listen to this guy.
It's a very decent sci-fi movie, we need more like these and less retarded tentpole politically driven Disney garbage.
If only the Cloverfield Paradox was this good.