We don't know. The only way we have to explain it is as the result of
a "Doppler effect", that is, by assuming that the star is approaching us.
But if the Doppler effect is really at work in the cosmos, then the vast
majority of galaxies, along with all of their stars, are receding from us ...
as if space itself and the entire universe were expanding !
That's the current assumption, because no other plausible cause
has been found for the shift of spectra from distant objects. But
nothing in science is ever settled, and the search goes on.