#1 - there were two L-39s and an L-29, in formation. They are also turbojets, so you have three additional sources of airflow interference.
#2 - how close did you fly to them?
Here's the ADS-B tracks, the accident plane got there 10-15 seconds after the three jets. I doubt you got that close.
ADS-B Exchange - track aircraft live - aircraft flight history
globe.adsbexchange.com
It doesn't take much to destabilize a slow plane making a base to final turn, and here you have three jets going at 150 kts, 300 feet above him, no more than 600 feet to the right (no idea how the formation was spaced and the track only shows one aircraft), turning and throwing their exhaust and wake turbulence in the path of the accident aircraft. Even if the wake turbulence didn't directly cause it, the extremely tight spacing provided by the controller is still unacceptable, especially with the aircraft involved.
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