American Airlines began the race to charge checked bag fees. First it was a second checked bag fee, then fee for the first. It became industry standard with Southwest remaining as the outlier.
With passengers charged for overhead bin space they began carrying more onto the plane. With planes largely full for the past several years there hasn’t been enough bin space to accommodate everyone, leaving the last passengers to board having to gate check their bags usually meaning that they must collect those bags at baggage claim.