It's the 7 year
economic cycle that you've been told about. Aviation is said to be a good barometer of the world economy so airlines' fortunes, and thus recruitment policy, would follow a 7 year cycle.
The 2001 to 2008 cycle is a good example. Both autumn 2001 and autumn 2008 were bad times to be seeking work here in the UK, whereas 2004 to 2007 were excellent.