Some subclass are based on the idea of mixing classes together. Eldritch knight is a fighter with wizard spells. Hexblade is a warlock with more of brawler focus like a fighter. Paladin oath of the ancients is a Paladin mixed with druid. Every class has a sort of hybrid option. Hence hybrid class is how I see them. Some are very strong, others not so much. I can list the phb ones if you want.