I read about it and I thought: nah, can't be that obvious. But it is, that's a fascist salute (adopted by the nazis, but you can see it in other fascist parties).
There's no excuse, it is a premeditated gesture, IMHO. He's going from his heart to the crowd and could have done the same with his palm up, as that's how you share something (love I assume in this context of thanking the crowd); not with the palm down.
Probably a conscious provocation.