A tragic accident.
In my experience, close formation, at least in daylight, can surprisingly be less risky than a wider tactical formation.
The former leaves the crew in no doubt of the proximity of the other/s and requires full concentration on it. The formation leader takes much of the responsibility for terrain clearance and the radio. It has little practical use in a battle situation.
Tac formation allows more freedom of movement and each aircraft can move around to some extent and individual crews become fully responsible for terrain clearance and lookout for other aircraft, ground threats etc etc. One can temporarily lose contact and awareness regarding the exact proximity of others, especially during or after a turn, where crossovers from one side to the other of the leader may be required.
I recall one instance where I was leading a low level tactical formation in radio silence when another, identical type suddenly appeared and crossed from left to right ahead of us and I had to take very rapid avoiding action. When we had recovered our composure we realised it was our formation number two! There but by the grace of God….