It seemed like a relatively straight forward sequence when it was explained in the video
Passing directly underneath another aircraft for a station change is actually quite difficult. "Normal" station changes from echelon to either line-astern or opposite echelon have the aircraft drop down and back slightly before moving laterally.
When you pass directly underneath, there is limited scope to assess vertical closure, which
appears to have happened in this case. Certainly not something you'd try unbriefed.