What is the power source for each?
Druids = nature (or nature gods or nature spirits)
Clerics = divinities or the powers behind them
Warlocks = demonic
Wizards = the science of magic (?)
Bards = ?
Creativity, inspiration, ideas, reason. The collective unconsciousness. The divine fire which Prometheus stole and gave to humanity.
Maybe there's just inherent magic in groups of intelligent beings, like there is in nature, and that is what the bard is tapping into.