It's natural for anyone to want to tell something that they know. It is like a human instinct. Especially, if it something that will affect someone else. It is a natural feeling to tell at least one person. Yes, sometimes that person is the wrong person to tell. However, it is still human nature.
Sometimes, it comes down to morals, and doing what is right. No matter how mad the person gets for their secrets being told. It may be better in the long run, if someone else knew what was going on.
Gossip is a big fad. People like to know things about other people. Maybe it makes them feel better. Maybe its because they judge everyone. Maybe it is so that they will actually feel better about themselves.
Habit. Sometimes, people just have a habit of telling everything they shouldn't. It is just the person they are. Not that they necessarily want to tell things. It's just they don't know how to control themselves from stopping it.
The news is just too much for them to handle. Yes, this can happen. Sometimes, knowing something can eat away at a person. Even if it is good. They just can't keep it inside, knowing that nobody else knows. It's like knowing someone is having a baby, its hard not to be excited, and not tell anyone. At the same time, its like knowing someone stabbed someone else.