Prison riots, Detention center riots and private security firms
They have a lot in common and its not just Manus Island.
When governments shelve their responsibility and out source to private security firms they get what they pay for and they don't get much for their buck. They are in the business to make money and that means they cut costs, be it training or staff numbers, that's how they make a buck. As for the training it appears that almost anyone can qualify.
Governments just turn a blind eye because it's cheap.