Nesikep said:
from the article i saw it lost 2800 ft in 30 seconds, not 12,000.. it's still very fast decent, it's an order of magnitude less.
I heard of storms in the area, maybe they flew into something that damaged the plane? Unless they stalled it (and there's plenty of warnings when that's going to happen), it's hard to imagine what else.
That descent rate is less than 2X what a normal rate of descent would be for that aircraft, but what is missing is at what altitude the abrupt uncontrolled part of descent began with the time factor versus the forward air speed..not the same as acceleration.
From the flight data stream at Flightradar.com, it looked like a normal descent from 40,000ft to 20,000 ft. It made the transition thru 14,000ft normally, and according to the graphic below, flew level at about 6000 ft for a short period of time.
[UPDATE 7]
According to the NTSB, the aircraft was on a standard arrival route from the southeast. At 12.30 local time, the pilots contacted Houston Approach Control as they were descending through approx 18,000 feet, about 73 NM southeast of the airport. ATC advised there was light to heavy rain ahead and provided vectors around the weather. At 12.36, flight 3591 was cleared to descend to 3,000 feet. At 12.39, when the aircraft was at approximately 6,000 feet, contact was lost with the Boeing 767 Freighter.
You can follow the flight from Miami until it crashed here:
https://www.flightradar24.com/data/aircraft/n1217a#1f98a1ae
(times are UTC which means you need to subtract 5 hours from the time in the graphics above to get Houston time)