I assume it was turbulence, but that's sort of a general term, a catch-all phrase for a plane shaking all to hell for a variety of atmospheric reasons.

Then I read this article and, although it doesn't have me putting on my foil hat, it does raise more questions than it answers.

Some snippets:

The plane dropped 178 feet (54 meters) in less than one second, which “likely resulted in the occupants who were not belted up to become airborne” before falling back down, the Transport Ministry said.

(Adding that for factual information. I seem to remember early reports of it dropping 3,000 feet or something.)

Early findings show that as the plane cruised at about 37,000 feet over southern Myanmar, it began to experience slight vibration due to changes in the gravitational force, the ministry said. The jet’s altitude increased — likely caused by an updraft, not by any action of the pilots — causing the autopilot system to push the plane back down to the selected altitude, the report said.

A lot going on here. I assume 'gravitational force' is a standard term in these reports, but I've never seen it before. But the plane's altitude first increased, then decreased via the autopilot system.

So we have an updraft... but then the plane plummets. But wait, there's more.

The pilots also noticed an uncommanded increase in airspeed, which they tried to check by extending panels called speed brakes, and “a pilot called out that the fasten seat belt sign had been switched on.”

An uncommanded increase in airspeed? Do they mean even by the autopilot? Very murky details here.

Finally:

It was unclear what caused the turbulence.

So, basically, they lost control of the plane and don't know what the fuck happened.