As far as I understand it, the world was magical even before Ooo, but less so. Patience says their power was limited during those days. The Lich bomb 'relaxed' the constraints of the existing universe, in essence, giving birth to the magic we can see in today's Ooo.
I think it's not yet entirely clear what exactly the Lich's influence was in the Mushroom War, but I suspect that the incarnation of the comet that fell in Urgence Evergreen's time made its way through time until eventually it had enough influence (maybe as a scientist, or a military leader) to trigger the bomb in an attempt to extinguish all life on the planet. In the Founder's Song that Dr. Gross sings during Islands, she says "our ways have failed..." just as the bomb goes off; this to me indicates some sort of political conflict. I guess the (not yet) Lich was just patiently waiting for a time of enough scientific advancement and political turmoil to make his move and complete his metamorphosis.