Why do so many mass murders prefer the Bushmaster .223?

This article in Slate notes that the Bushmaster .223 AR-15 is the weapon Batman movie shooter, James Holmes; Oregon shopping mall shooter, Jacob Tyler Roberts; Newtown Connecticut school shooter, Adam Lanza; and Webster New York fire fighter assassin, William Spengler all used in their mass murders. Counting the shooters who did themselves in, the toll from these four crimes stands at 46 dead and 63 injured.

In most instances, the shooters had other arms as well, but the Bushmaster AR-15 happened to be the weapon responsible for most of the deaths and injuries in each case. Why the Bushmaster? Is it just a 2012 coincidence, the result of some advertising campaign or Web graphics that inadvertently appealed to people plotting mass destruction, copy-cat syndrome? What explains the sudden surge, in 2012, of mass shootings using a Bushmaster .223?