This is a copy of a very similar incident that happened in Baltimore in 2004. It completely rewrote the rules for performing stability tests on pontoon boats, and indeed, caused the Coast Guard to reevaluate the weight of Americans, effecting the stability calculations of all inspected vessels. The Lady D was significantly smaller than the vessel in question, but didn't carry the dive gear, either. US rules state that, for every diver you have on board, you must reduce the number of passengers by 1/2, as they assume each diver carries 92.5 lbs of gear.
http://www.ntsb.gov/doclib/reports/2006/MAR0601.pdf
One of the things that can happen to pontoon boats that don't happen to true catamarans or monohulls is that if the boat is in a following sea, it lifts the stern and buries the front of ONE pontoon. If both pontoons bury, there is enough reserve buoyancy to bring them both back afloat. In the case of one pontoon pitch poling, though, the pontoon wants to keep heading down. Anyone who has pitch polled a hobie cat can attest. Keep the pontoon on the surface, life is good, but let it go under just a little bit and it's bad. A true catamaran has hull between the hulls, so even if you bury one side, the deeper you go, the more buoyancy is generated. With a pontoon boat, there is a platform between the hulls, and if you get it in the water, it works to drive the pontoon even deeper.
I wasn't there, so I can't comment on specifics, but I'd guess that they took a following wave not exactly on the stern. It raised the stern, but not squarely, pitch poling the boat.