This is the third time I got excommunicated for no presumable reason in 1444. It is seriously hindering my ability to retake my Cores. Is this random or basically an event that always happens?
You can't be excommunicated if the papal state has positive opinion of you.
The papal controller can excommunicate any catholic nation that the papal state has a negative opinion of. Fortunately you can just improve relations with the papal state to avoid this as you can't be excommunicated if the papal state has positive opinion of you.
Or, you can drop Provence and ally the Pope. :)