Roger Pearse is currently blogging (1, 2) on the origin of the story that Caliph Omar ordered the destruction of the library of Alexandria. There are several other stories: the library was destroyed when Julius Caesar captured the city, it vanished in the 365 earthquake, Aurelian‘s soldiers are responsible, Christian agitators set it afire.
But perhaps it is not necessary to look for such an event.
There are several reports about the number of scrolls in the library. 400,000 is one of the lower estimates. Now a papyrus scroll is vulnerable and a book needed to be copied after a century. This means that every year, about 4,000 scrolls had to be copied. If we assume that one scroll took one month, we need about 300 to 350 writers, excluding the correctors, illustrators, the people who prepared ink and papyrus, and so on. All these people were highly paid professionals.
These numbers are all guesswork, but they serve to illustrate a point: the library was too big to survive. Even if Caesar and Aurelian hadn’t attacked the city, even if there had been no earthquake, even if there had been no interventions by Christian and Muslim fanatics, the library would have vanished.