Some dates are changed to try to celebrate the feast on the day of death; others are moved around because of a concern about celebrating too many feasts in, say, Lent.
Some changes are just deplorable. Philip and James were observed on 1 May for centuries; Pius XII moved them to what was then the next free day, 11 May, and then Paul VI to what was for him the next free day, 3 May…though the best choice would have been to give them back their centuries-old day.
Matthias was also on 24 February for centuries; it’s almost always Lent, so the decision was taken to pick 14 May, though that was more or less simply pulling an Eastertide date out of thin air (because of Matthias’ connection with the immediate post-Resurrection period).