They said they wanted to avoid a situation like last year where public outcry made them change their mind on firing him, especially if he was to magically pull a few games out of his you know what like he typically does. This was long overdue, even in their championship year he managed to lose 2 games. They've been winning in spite of him for years.
yes, they actually fired him now because they're worried that Miles will pull some miracles out and manage to hang on for a little longer if he was to continue to stay on. Funny, but they were actually afraid he'd do too good record-wise.