This is way too harsh.
I saw no anger.
Maybe frustration.
Look at other examples of Nalbandian, Shapalov, and even young Tim Henman, who clearly reacted in anger and took a huge swing and hit someone.
In this case it was barely hit: it's unfortunate it hit the old lady in the throat, but a game penalty would have been enough punishment.
EDIT:
After seeing the whole incident, with the lady instantly falling to, the ground, i have to change my mind.
Compared to what Shapalov, Henman (it was so bad youtube censors it even today) and Nalbandian, it didn't look at all worthy of DQ. But even though the ball wasn't hit very hard, it hit the vulnerable throat spot and the lady was instantly affected.
This once in a lifetime fluke couldn't have happened at a worse moment for Novak, right after he created his association, and it will leave him vulnerable to critics. I hope it doesn't cause it to fold.
It's a bit like accumulated bad Karma for screaming and shouting at the crowd culminating in this payback.
It's as bad as any heartbreak that happened in his career, possibly even it's worst.
What a crazy, crazy year this turns out to be.