New 3 Man Mechanics?

[Steven Tyler]

I saw this in an American Association game about two weeks ago.

No runners on and batter hits a hard ground ball between the bag and F5. U2 points the ball fair and turns as the ball is rolling down into the LF corner. U2 then immediately starts chasing the ball down into the corner. U1 comes in and pivots and takes the BR into second. HPU just stands at home.

My question is why did U2 chase the ball down into the corner and HPU stay at home? I don't recall ever seeing an umpire go out on a ground ball before.

I suppose the crew reverted back to two man when the U2 went out and U1 would have take the BR into third. Should HPU have been at 3B?