Logical? They capture humans to dip them into the FEV and expand their army, but also eat them and use their gory intestines to decorate every place they come across. They are all aggressive for some reason, which is never really explained. What are they even after?. You come into their only breeding ground, yet there aren't really so many mutants there at all, and they can't seem to obliterate a settlement full of children. And all the intelligent mutants, instead of leading the rest of the bunch, live as loners.
They aren't logically part of Fallout lore, they're there because Bethesda needed some more "awesome" things for us to shoot at.
And the talking Deathclaw in Navarro tells you about their origins.