The best way to describe it would be something like this.
High school boy with pimples asks a freshman gal out for a date.
The gal tells him to go to hell.
High school boy eventually wins her over his junior year, as his acne has cleared.
He gets his finger wet.
She then dumps him.
He then starts spreading rumors about how many guys she's slept with, that she has crabs, and that she's bat-shit crazy.
The 5 balls in the water on the 15th was basically Sergio reminding everybody that although he got his "finger wet" last year, the "gal" is still bat-shit crazy and has crabs.
Except now that he's a Green Jacket winner - he's expected to maintain the tradition, which (for him) means keeping his mouth shut, not complaining, and smile and nod when needed.
But deep down he absolutely hates this layout. Still.
His 5 balls in the water yesterday was the only way he could outwardly demonstrate that this isn't his favorite course.