Because they're all over-the-top personalities and Steve was really, really annoying, annoying enough to trigger an intense reaction. It was a perfect storm. Even JESS was willing to bash his head in.
Actually, also, it's a commentary on how utterly essential Diana is to the team. Diana is their anchor, their rock. They don't function at all without her. She's got both the drive to be a hero and the experience and skills to perform her duties to the utmost degree. She's the perfect teammate and the perfect leader. Without Wonder Woman they fall apart. All of them instinctively realize this, so when something shows up that compromises or undermines Diana, they rightly view it as a threat and want to eliminate it.