This is obviously a difficult situation for everybody. I agree that it is best to fire someone face to face, and that is how I have always done it.
On the other hand, A) Calling Banker and telling him to come back to Lincoln would have resulted in him asking why, which leads to the same result; B) Since Stewart hadn't officially left yet Banker had to go to create room for Diaco; C) I suspect (with no evidence to back this up) that the initial discussions probably took place during the AFCA meetings this week, and Riley had to strike while the iron was hot; and D) If Riley waited to bring in Diaco for an interview until he had brought Banker back and fired him in person, Diaco might be the new DC for Arkansas right now.