As Casspi was going down the lane, Nowitzki, giving chase swung his arm in an attempt (so we believe) to block the shot. Instead, he drilled the head of Casspi who went flying into the safety pads on the basket and into the cameramen in the front row.
It didn’t appear to initially be dirty, but it was a hit to the head which resulted in Nowitzki being charged with a flagrant two foul.
Now, according to Chuck Cooperstein, the NBA is reviewing the foul which could lead to a suspension for Nowitzki on opening night:
Mavs holding their breath a bit today. Flagrant 2 by Dirk being reviewed by NBA. Could be suspended for Opening Night. Emphasis on could.
While the NBA does take blows to the head seriously and will suspend for them, there didn’t appear to be any ill-intent here and it was just a basketball play that ended up in a scary scene. I doubt Dirk sees anything more than a fine.