All we know so far from the report;
As with the previous attempt, Viper 2 moved rearward to allow Viper 1 to roll inverted. As Viper 1 stabilised in the inverted attitude (panel A in Figure 4), Viper 2 began manoeuvring to pass beneath Viper 1 (panel B in Figure 4). Viper 1 then stabilised in the inverted attitude and as Viper 2 approached, the vertical separation between the 2 aircraft reduced. Viper 2 passed beneath and began to pitch up and bank away from Viper 1 (panel C in Figure 4). At 1333, as Viper 2 climbed and banked left, the right wings of each aircraft collided (panel D in Figure 4)..
From that, and the images it seems V2 simply pitched up too early before they were clear and made wingtip contact, it mentions banking left, but it had hardly started that from the pictures. The pictures seem to indicate V2 started from behind abeam, meaning V1 had limited sighting, probably even less so hanging inverted. So V2 moved from behind abeam, underneath and then pitched up and a shallow bank as they moved from underneath.
On the face of it, it sounds crazy that if you’re underneath and looking up that they could have even come close to the inverted one, let alone hit it.
That might be part of the issue, the pilot of V1 is probably 100% focused on flying inverted level, with the safety on lookout, no external distractions to his flying. V2 has a camera operator/safety, who is filming, so the pilot is flying the manoeuvre and looking up for the separation aspect, we all know the body follows the eyes/head movement, ie you go where you look, if you are not careful, so it is very easy to impart rearward movement on the controls each time you look up, especially considering how far you'd have to tilt your head to observe an aircraft directly above you.
I've done similar manoeuvres underneath aircraft in formation, albeit much slower machines and not involving an inverted aircraft, and were quite conscious that the lead was maintaining straight and level ahead, as they had no sighting on us, and that we had outs, as well as moved well clear before attempting movement towards/through the targets level.
It might just come down to a 'went off plan' accident, where everything to be done was briefed and discussed and then V1 has decided to vary the play book and a few critical things were not thrashed out.
In any case it looks like the ATSB has a lot of information on this accident, and the final report should be fairly thorough with lots of answers.