You and your wife/boyfriend/girlfriend find out your going to have a baby. Isn't this great? It's a confidential secret between the two of you and your Dr. Of course, you have to start seeing some 3rd party people, such as the sonogram technicians and whatever. You find out it's going to be a girl.
You pick a date. You invite your parents. His/her parents. Your friends. His/her friends. You buy a baseball full of pink confetti to do the gender reveal. You're all set. This is going to be great.
Then, someone goes onto your facebook page and congratulates you on your future daughter. They found out, through whatever means, and felt they doing good by serving all your friends and family by sharing the good news.
Now, was everybody going to find out anyway? Yes. Did this person !@#$ everything up by taking ownership of the reveal and saying it? HELL YES.
Granted, this is apples and oranges and they announced Ghostbusters and stuff. But if you found out that people going to the same OGBYN as you and your significant other were having their news revealed through social media would you feel the same way? Would you run away from them and find a new Dr? Would you go back to them if/when you are blessed with another child?
These people see their creations as their children and invest years of their lives working on them. It's not so hard to see why someone would get pissed over something that some might see as "insignificant" while others see as their offspring.