Game Management or not?

[Andy]

Situation presented to me from one of our local HS umpires:

Multiple runners on base, base hit to the outfield.
Ball is returned to the pitcher in the circle, BU starts moving to position, PU calls time and requests to talk to the BU.

PU's question to BU: Why didn't you call the look back violation on the batter-runner? That is your call.
BU: I didn't see that, I was watching the lead runner.
PU then proceeds to declare the B/R out on the look back violation.
As a result of this, the third base coach protested enough to be ejected.

1. Shouldn't the PU have just called the look back violation when he saw it?
2. What is your opinion on making that call, even though it may have been the right call, after that delay and conferring with the BU?