In my campaign, I've arranged for the Titans to be beings of a former Time. By Time (with a capital 'T') I mean before this Time began. I got this Time idea from some board here before and ran with it, integrating the Hindu notion of death and rebirth (and they believe that the universe itself dies and is reborn...I simply fed this over to the notion of Time).
I'm not trying to be confusing, but I pictured an entity (a Titan) during the previous Time in which Time itself was bestowed upon him (as many other qualities and traits were bestowed upon others). When they left or did certain things, these attributes of the world/Time suffered. However, when the Time Titan decided to leave, it caused a massive rupturing in the space-time continuim that spilled the two realms (heaven and earth) into a near infinite set of realms. In this apocalypse, most were killed. However a few Titans managed to survive, including the Time Titan...
...and he's just been unintentionally released by the PCs...
...and the new God of Time isn't too happy that the Time Titan (pardon the bad names) is now free. And no one is sure of the consequences that two entities with power over time are free.
There are plenty of other little odds and ends, but for me, the Titans are the races of the previous Time.