Some airlines allege 'extraordinary circumstances', e.g. to deny liability and shirk compensating under Recital (14) of Regulation (EC) No 261/2004:
(14) As under the Montreal Convention, obligations on operating air carriers should be limited or excluded in cases where an event has been caused by extraordinary circumstances which could not have been avoided even if all reasonable measures had been taken. Such circumstances may, in particular, occur in cases of political instability, meteorological conditions incompatible with the operation of the flight concerned, security risks, unexpected flight safety shortcomings and strikes that affect the operation of an operating air carrier.
How can passengers cost-effectively check if the airline's telling the truth, without and before a lawsuit? Doing so after the lawsuit looks foolish. as:
a judge may not grant you a subpoena/witness summons.
if you learned that the airline were truthful only after filing the claim, then you'd lose and must pay legal costs (e.g. in England & Wales).
the airline can falsify its records without a layperson's knowing it.