Eindhoven airport does not have airbridges. As distance from apron to the terminal is short all pax need to walk to and from aircraft. Even when it rains. Except ofcourse wheelchairs.
This local newspaper reports that when all pax had boarded, after ten minutes the engines and airco were started. This lasted for a few minutes and then airco was shut off again.
Passagiers vliegtuig Ryanair oververhit - Airport - Specials - ED
Steps could be used so doors could be opened withour risk of pax falling out of the aircraft. No idea why this was not done.
The only thing I can think off is Ryanair wanted to save fuel by not running engines. And as the flight was already delayed wanted to blame the handling company for the further delay as pax were offloaded at the end.