[umpjim]

So we may have this:

"Rule: 3-2-3



ART. 3 . . . No offensive team personnel, other than the base coach, shall be near a base for which a runner is trying so that a fielder may be confused; nor be on or near the baseline in such a way as to draw a throw; nor shall the base coach or members of the team at bat fail to vacate any area needed by a fielder in his attempt to put out a batter or runner.

If a thrown live ball unintentionally touches a base coach in foul territory, or a pitched or thrown ball touches an umpire, the ball is live and in play. If the coach is judged by the umpire to have interfered intentionally with the thrown ball, or interferes in fair territory, the interference penalty is invoked.

PENALTY: The ball is dead immediately and the runner is out. The batter-runner or runner may be out as in 7-4-1f and 8-4-2g. Other runners return as in 8-2-8. "

While interference with a thrown ball requires intent, interference with a fielder trying to put out a runner does not. Maybe the coach not only did not vacate the area, he put himself in it. Or he interfered in fair territory.
Just guessing about what really happened.