Circular reasoning in the explanation of red shift?

I apologize if this question seems trivial, but I have been troubled by this issue for quite some time. The redshift observed in the wavelengths of the light from distant galaxies is a significant piece of evidence regarding the expansion of the universe. The reason behind this observed redshift is the relative motion of light, the fact that space itself is expanding, right? Isn't this circular reasoning? Evidence for expansion? Redshift. Reason behind the redshift? Expansion.

I would greatly appreciate it if someone could elucidate the relationship between these two concepts.