Penalty question

[jimpiano]

This is not to dispute a ruling, but to ask for the appropriate rule.

A forward pass is thrown toward a wide receiver near a sideline.
Before the pass gets to the receiver it is tipped by a defender and sails high and uncatchable out of bounds.

A second after the tip and deflection a SECOND receiver, a few yards ahead of the receiver, who makes no attempt to go after the ball, is hit, hemet to helmet by a defender.

15 yard penalty against the defense was the ruling.

What rule covers this?

I thought the officials got it right.

Just would like a rule interpretation.