Phileas
It was not just Air Europe that went down this route in the 1980s. BA substantially increased its European nightstops around this time. Nowadays a large proportion of the LCY operation works on this principle.
The reasons are clear. The early morning inbounds to London are the strongest traffic flows, and in the case of continental Europe there is the added factor of the time change to take into account.
EasyJet's domestic services to the London area airports are all based on making the first rotation a London inbound though of course they originate from other EZY bases. This did not happen at SEN and this could well be a factor in the relatively disappointing performance of the SEN domestic routes.
Assuming an airline has assessed its market correctly, the decision to start the day from a European nightstop can be a no-brainer.