Kind of everything...
Okay, make it simple for you. Up until the 1970s, security on airlines was lax. After a bunch of needy people hijacked some planes to Cuba, they tightened up a bit. After some really evil people crashed some into buildings, they really, really cracked down. (Although if they listened to security experts earlier, that never would have happened.)
Social Media makes it easy to organize a flash mob. Now the flash mobs have figured that while stores are capable of dealing with single shoplifters, they can't deal with a mob.
So. Get rid of the vulnerabilities, just like the airlines did. No more open windows that can be easily broken, secure the entrance points, use better intelligence to identify the flash mobs.