How only 208 people can cause an overload? it is less than 20 tons of aditional weght on 774 ton ship. If less than 20 tons of people moving around can contribute to capsize then design of that ships is seriously flawed.
You would be surprised. I use to sail a 1,500 kg sailboat. 750 kg of that mass is located in the keel, which goes 1.5 meters underwater. And the position of the crew (2-4 persons usually) is essential to keep it's roll angle to 15°, the position where the hull has the best hydrodynamic performance.
On a 56 years-old riverboat, designed and built just after the war under the soviet era, with poor maintenance and exposure to the russian winter (56 winters during which the hull bends then takes back it's original shape under the enormous pressure of the ice), I have no doubt that if the 200 passengers all stand on the same side, during a storm with nasty waves (riverboats, especially old ones, are not designed to deal with waves as they are supposed to float on still waters), the ship can capsize quite easily.
This is a matter of keel. Riverboats have no or a light keel in comparison to sea boats. Also, the shape of their hull is "flat", in order to allow them to sail in shallow waters. This design is practical on still waters, because it allows to carry more people or goods. Also, the ship will not roll.
The problem is, that if serious waves show up (and that can be the case on a wide river during a storm), once the ship exceeds a given (rather small) angle of roll, the ship will capsize and there will be nothing to do to save it. The drawing posted by SiberianTiger shows that well.
With a boat designed to endure sea conditions, like the sailboat I use, but also like cargo, freighters, warships, fishboats, a big angle of roll can be endured without damage. Thanks to the mass of the keel, the ship will naturally tend to keep a roll angle of 0°. The drawback of this design, with motor-powered ships, regardless of their size, is that they tend to roll from one side to the other. This can get terrible when the weather is bad, and the passengers, if any, usually get sick quickly. Modern cruise boats are designed to be a compromise between stability and safety. With sailboats, the pressure of the wind on the sail allows to keep the roll angle on the same side, which is much more comfortable.
This can be extreme. On a few occasions, we got surprised by a gust of wind and the tip of the mast hit the waves (roll angle > 80°). But the ship did not capsize, thanks to the keel that is 50% of it's mass. The real danger is for the crew, that can fall on water since the deck suddenly becomes vertical (it's why you should always wear a life jacket and a safety line on a sailboat) !
In the case of the "Bulgaria", a small roll angle might have been enough. I also suspect there could have been some residual water in the hull, which is the worst thing in these situations.
