Anybody who has been responsible for getting airside passes will know it will take many weeks from job offer to getting boots on the ground. Criminal record checks can take many weeks (not sure what current waiting times are, somebody here will know), getting 5 years references for staff who may have had several or many jobs over that period can be a nightmare. Then all the paperwork has to be vetted by the ID centre, eventually the new employee will have to wait to be called in for their pass. Finally a training period before the security agent or baggage handler can be productive.
So it looks as if there were some serious shortcomings with the HR functions of the airport operators and handlers. The recruitment process needed to be started at least 2 months ago to have anything like a reasonable number of trained staff in place for the well predicted rise in pax numbers.