Hitting them THAT hard down the rail, he could also be testing his stroke to make sure it is straight. Shots like that need to be struck very accurately, especially hit very hard. If they're going in he knows he is stroking straight through.
That's close to my assesment...
I warm up by hitting hard, long shots and not trying to focus too much on them... mostly to loosen my arm up. Then tone it down to dial things in. No sense in trying tricky cut shots when you are totally cold... won't warm up your arm or your mind.