The FAA standard weight for passengers is 170 pounds (175 pounds in the winter where people may have additional clothing). That's the planning weight the airlines assume as average for each passenger. While it was accepted that some passengers would weigh more, it was assumed that enough passengers would weigh less (especially women) that the average would be reliable. This limit has been around for a long time.

Unfortunately, Americans have gotten bigger over the years. According to a news report from a couple weeks ago, some 16 of the passengers were men. Odds are the average passenger weight was well over 175 pounds. In addition, they assume the luggage is going to weigh about 35 pounds per bag, which may not be a reasonable estimate. I don't know what the assummed weight is for carry on luggage.

The Beech 1900 that crashed was estimated to be within 100 pounds of the maximum takeoff weight using the old passenger and baggage weight assumptions. The plane may well have been overloaded. In addition, the distribution of the weight was near the aft center of gravity limit. Had the plane been overloaded, especially if heavier passengers sat in the rear seats, and the aft CG limit might have been exceeded. That would make the plane very hard to control.

Now, they know this particular plane was experiencing some sort of elevator (controls pitch) problems. I have not seen what kind of pitch problem it was having, but if I had to guess, I think it might've been in the electric motor that controls the elevator trim. Sometimes these motors act up and cause uncommanded pitch trim changes. Combine that with a possible tail heavy condition and possible overloading and you've got a formula for disaster.

According to the data recorders, the plane pitched up very steeply after takeoff. Instead of climbing out at a normal angle of about 7 degrees, it tried to climb out at over a 50 degree angle.

Another potential complicating factor is possible if the baggage shifted to the rear during that steep climb. That would've made the plane even more tail heavy. The crew surely tried to push down on the elevator, but as the plane climbed, it would've lost airspeed, reducing the control effectiveness and making recovery impossible.

Most accidents are caused by a chain of events. Break the chain at any point and you prevent the accident. This accident sounds like it follows the pattern.

In addition to trying to find out the actual weight of the passengers, the investigators are reevaluating whether the average weight needs to be increased for both passengers and baggage. This is especially important for smaller planes where there's less margin for error.