The falling of the chandelier was not, as what was long believed, to be Erik's fault. The chandelier was held up by eight counterweights, each of them at 1,700 pounds each, making the chandelier weigh aproximately six and a half tons. The counterweights were used to balance the chandelier rather than hanging it directly to the ceiling so that it could be lowered to be cleaned (I think.. that's what I read). One of the chains holding the counterweights became rusty and during a performance, crashed through the ceiling landing on the head of the new concierge (in other words, the woman who was to suceed in managing Box 5), killing her instantly. In chapter seven of "The Phantom of the Opera", is states "'Two hundred kilos on the head of the concierge.' That was her sole epitaph!" When the counterweight fell, the chandelier became heavier than the remaining counterweights and fell.
Other Information:
The chandelier was designed by Charles Garnier, the man who designed the Paris opera.
Also, in the new movie, the falling of the chandelier had been moved to the end. There's a line in "Masquerade" that they had to change. "To a new chandelier!" Why? Because the chandelier hadn't fallen yet.