Doesn't surprise me that much to be honest.
Draymond Green has the "Warriors glow" right now. What I mean by that is anybody who plays for the Warriors gets a free pass, and everybody just assumes they are perfect humans who poop gold.
It's obvious from watching Green play that he is a very passionate guy (much like Demarcus Cousins, KG, etc) and when you play the game with that much passion, it's natural to assume that he's going to have times where things are not going to go his way and he's going to vocally express his frustration.
Even the most calm and collected among us lose our cool and snap sometimes. We as humans can only take so much. Who here can honestly say they have NEVER snapped and lost their cool?
The only difference is that these guys are pro and so they are always in the spotlight. If you lose your cool once in 3 years, that's all it takes for the media to be all over you like you're a psychopathic serial killer.
Guys like Draymond Green HATE losing more than anything. For a guy like him (who defeated odds with hard work to get to where he has) you probably have a massive chip on your shoulder and take things even more seriously than most, because nothing came easy. It's not unusual to imagine him venting some frustration once in a while.