When a runner tags up from a base on a flyout, why does the fielding team need to tag the base to find out if the runner tagged up too early? I mean why doesn't the umpire make the call automatically. I mean the umpire needs to already know this is the fielding team tags the base and asks. Why is this the only play (that I know of) that a umpire won't make as pasrt of their umpiring job?

Thanks

Not even close.

When the fielding team throws the ball to the base they are making an "Appeal Play". In this situation, they would be appealing to the umpire that the runner left the base to soon.

Now for the Plate Umpire. He is supposed to position himself/herself so that he can see the first touch by the outfielder and the runner. If the ump sees the runner leave the base to soon, it is nothing until the defense makes the appeal. In order for the ump to rule on the infraction, the ump must see the player leave the base early. If he does not see the player leave early then the runner is safe.

Years ago, there was no appeal necessary. If we saw the runner leave to early, we banged the runner out. However MLB decided to change the rule to require the appeal.