building works
Sorry to state the obvious....
Increasing passenger numbers = facilities no longer adequate = expansion = building works.
Static (or lower) passenger numbers = adequate facilities = no expansion = no building works.
Most major airports have building work going on somewhere. Hopefully the results at Luton will be worth the current disturbance. Airports that used to be operated by the state-owned (and taxpayer subsidised - please don't take this as a cheap jibe at anywhere in particular) British Airports Authority were in the happy position of being able to afford major expansion whereas Luton has always had to do things a bit at a time.