Dual engine failure. Extremely rare. The fact they got out a Mayday indicates they knew something was very wrong. The standard rule is aviate, navigate, communicate. If they had time to get out a Mayday, they knew they were fucked.
The video of the crash shows they never lifted the gear, which is unusual since it's a huge drag factor. If one is going to crash, crashing with the gear down is good unless going in water since it adds a buffer on impact.
The flap theory would do it, but the Captain should have noticed the error and called out a correction, "flaps down, gear up". Hawkeye is correct; the black boxes, both the Cockpit Voice Recorder (CVR) and Flight Data Recorder (FDR) would give all of that information.
The CVR records from multiple microphones; all the headsets and a general microphone. The FDR records hundreds of data points including the basic ones such as position of the gear and flaps, engine power, altitude and airspeed.
Most accidents are have a human error element if not being the primary cause. Pilot error being a big one but also maintenance error, which is rarer. ATC error is a major cause around or on airports, which this one does not seem to be a factor.