Sounds like a sudden downdraft.
Don’t be silly. You know how much we’ve ruined the atmosphere with our greedy capitalist CO2 generation. Well, Bucko, CO2 is the heaviest gas in the atmosphere. Since our disregard for mother Earth has resulted in a 4 quadrillion ppm increase in CO2, the explanation is simple.
This was a carbon jet column of pure CO2 shooting down to the ground, because of gravity, and the plane just got in the way.
See, you capitalist dogs almost killed a planeload of innocent school children!
/sarc
This event was similar to an event that happenend to Emerates 231 in December. What they found was that the crew had not entered the correct climb altitude in the flight director (autopilot) during the pretakeoff checklist procedure and had left the setting at 0. Shortly after hand flying the takeoff they switched on the FD and the plane went into a steep decent to 0 altitude as it was commanded. They were able to recover quickly enough and did not reach the ground recorded by the UA 1722 flight. The training was probably a recheck of their preflight procedure to verify the altitude of the FD was properly set before switching it on.
There was turbulence at cruise altitude near Maui on the day of the UA 1722 flight but nothing at the surface. If they had hit a microburst they would have reported it immediately as a warning to other pilots.