There was an astonishing level of recruitment from 1978 onwards, which continued into the late 1990s. This has now worked through the workforce, and is now an imminent retirement bulge. Those joining in the late 1970s are now reaching their normal retirement dates.....Sadly, for various reasons, many are deciding to take an 'EG', thus compounding the problem.
Possibly, this relates to events from the 1950s onwards, when ATC recruitment was passed from The MCA, to The Civil Service, The Board Of Trade, The DTI, and then NATS/CAA. The sudden realisation that all those who became ATCOs after military service during or after WW2 were approaching retirement, caught the latter organisations out.