I think Firestorm has it spot on,
Jet2 tried to be too cute with the 60 - 70 Baby pilots they interviewed. Whilst the compass testing didn't endear the Company to most, being told "congratulations you've passed - but we have no jobs for you" was confirmation of what many had already perceived about the Company.
Their final mistake was to think they were the only game in town. Too late they've discovered most of their 'hold pool' have secured employment elsewhere.
If they're short of B737 captains this summer it wouldn't come as a huge surprise.
My sympathies are with the CP & Fleet Manager who seem to have been hamstrung by the HR monster at Jet2.