if you played d1 y1, your second subclass was unlocked after you reached a high enough level, believed to be your gaurdian becoming skilled enough at weilding the light that they could channel it into something different. We got our third subclasses in d1 from an outside source. Titans forged their hammers under the sun on mercury, warlocks... i don't remember specifically, you stood on a metal pole, got struck by lightning, and came out a stormcaller. ikora was there on comms, i think you rediscovered old records of stormcallers or something. as for hunters, well, cayde sent them to find a friend who went missing. instead, they found a corpse, with a bow. they took up the bow and avenged tevis, and everything was hunky dory until big man ghaul showed up and put a birdcage on our god. the first subclass was regained when finding the shard of the traveller and getting your light back, and after that the gaurdian would find fragments of items that related to the subclass. burnt-out hand cannons, melted hammers, cracked gauntlets, snapped bows, i cannot for the life of me remember what the warlock ones were, but you would find scattered remnants of your super, basically, and then charge them with light by doing public events or something like that. then forsaken rolled round and the traveller scattered seeds of light across our path, charges of energy that would inspire the gaurdian to focus their subclasses into new forms. as for the -blam!- subclasses we're picking up in september, my guess is we find some freaky artefact of the darkness and absorb its energy, or it was just in us all along, and we learn how to channel it. either way, i'm looking forward to hucking some scythes at some aliens. all i ever wanted.

Guardians powers come from the Light. Basically the Light is mimicking actual solar, arc, and void energies. Each subclass is based around different personalities. While Guardians can use multiple subclasses (and even obtain new ones from an outside source), they almost always tend to favor the one that best fits their personality. Zavala for example favors Sentinel, but has also been seen using Striker.

We all use the light. We Channel that Light with different elements. Our Subclasses are based on the personality of the individual but schooled/Disciplined the same way based on your personality type.