My guess is that they have grown confident nothing would happen to Donald Trump and they have allowed to security procedures to go unchecked. You know, it is not the same to protect the current president of the United States than protecting a former president...
Trump has had like hundreds of rallies during this campaign cycle in which there have been no major incidents, after the first few dozen of rallies we as human beings, start to grow careless of the official procedures (like clearing the roofs) and go straight to bodyguarding the VIP.
I don't have any doubt someone is supposed to be fired for this breach in security, specially because someone resulted getting killed by the shooter.