Search for the terms Gamma Ray Burst 775 and you will find a YouTube video on a suspected massive stellar radiation occurring around 775 AD.
I think it's interesting on a couple of levels, adapted to Ars Magica and Mythic Europe - that's very close to when Bonisagus was developing Parma Magica and Hermetic Theory. It could be that Bonisagus gained a bolt of inspiration at the time - even better if several other wizards of the time went mad from revelation or spontaneously Twilighted.
It might also inspire a more directly applicable event - suddenly, for no apparent reason, a wave of Vim vis washes over Mythic Europe, driving all manner of supernatural creatures to a frenzy and inspiring a wave of strong behavior from mortals. Crops, herds, and Vis sources produce a bumper crop, and astrologers claim that it was a once in every few century event.
Unfortunately 775 was 8 years after the founding of the order in 767... But in page 21 of the Theben Tribunal book, it says that is the year Tremere attacked the Theben League. Interesting interesting.