I heard a phone interview on Fox News with an investigator who was on the
scene at the time of the recovery. The Fox News interviewer asked about
the "loose plastic seats" that everyone is reporting, and the investigator
corrected her and said that the seats were similar to public park bench
seats, and were fastened down(still). He said that the seats hadn't slid,
but the passengers may have.
Asking the survivors if people slid to one side would be the way to find out
if the cargo mass shifting is what caused the capsize.
I suspect that the captain took the wake in a quartering move and this
caused a couple of severe lists that were exaggerated after the passengers
were thrown to one side after the first tilt. Crossing a large wake usually
results in two or three tilts.