Umpire over ball

[jTheUmp]

I'd also guess it was due to offensive substitution... the U stands over the ball to give the defense a match-up opportunity.

The rule says that the defense must not delay in making their match-up substitutions; however, if the play clock expires during this match-up opportunity, it's a Delay-of-Game foul on the offense (they should've made their substitution earlier).