Have heard twice now that Dan Rooney is the one that made the call on pulling the plug on Arians. I have no link, but it honestly makes sense. Tomlin says he wants Arians back, Art II says it's Tomlin's call, then the plug is pulled. The Steelers aren't some shady organization so Dan making the call makes sense to me.
Mike Prisuta is the source of this info so if you find him unreliable feel free to disagree but to me this explains a lot.