No, they wouldn't have jumped the gun because he was going to announce the VP pick on Monday. Unless that VP pick is Alex Jones or Steve Bannon, I doubt they would have done this timing intentionally. Trump's probably told the VP pick already that they got selected, and his inner circle probably knows too, so you can't jump in front of the announcement.

This may actually be a case of a lone gunman, with supporting incompetence from the SS & Police.