Intentional. He did it on purpose (based on the video)...
Disclaimer - I don't know who Tony Stewart is, or who the other guy is (Kevin Ward), or anything about the particular vehicle or track.
Did the man do it on purpose, like in trying to scare the other guy? or even trying to hurt him? Only he knows. But the blue car before him didn't hit Kevin Ward. And there was space to swerve left and avoid hitting him.
He didn't see the guy (because of lighting conditions)? Come on, give me a break. If they can see an object the size of a flag, they can see a man standing right there.