From the Aviationist:
The three Hornets remained in formation together until the merge point (where friendly fighters meet enemy fighters) and following the rules of engagement the two “MiGs” killed by simulated shots, executed aileron rolls to give the students a visual indication of which Bandits were killed and which one they should attack.
It was in the middle of the second aileron roll that Stubbs and Anderson collided, as explained by LCDR Stubbs himself to Rick Llinares and Chuck Lloyd for their book Adversary: America’s Aggressor Fighter Squadrons.
The nose of the Hornet flown by Anderson ripped through Stubbs F/A-18’s left wing and clipped off half of the vertical tail, while Anderson Hornet’s nose cone along with his canopy and his drop tank were lost. One of his engines was damaged as well.