Something very similar happened two weeks ago to my son leaving from here: his flight was delayed by one day. The reason was quite surprising to me: one of the cabin crew didn't show up and therefore, after a few hours waiting, they had to cancel the flights of 14 random people because of "safety issues". I guess that for each cabin crew missing, a certain number of passengers must be denied flying because it is impossible to attend the evacuation for everyone.
Results: 500 euros in his bank account a few days later. Iberia Express this time. Not bad!
Let me add that Iberia has improved quite a bit in recent years (and I fly with them regularly). I remember a terrible fight with an extremely rude checkin attendant years ago at FCO (Rome). But lately they have been improving. A lot.
Flights are always on time, and although they don't have the kind of faux mannered courtesy that you see in other countries, they do solve your problems in the end.
Which is what counts.