Players found a glowing meteor in a smoking crater. Energy from it unlocked their psi potential. Or a ship, a critter-ship, etc. Or some artifact deep in a dungeon. Like the monolith from 2001, basically.
PCs bathed in one of those Mind Flayer breeding pools. Were captured by a mad wizard and experimented upon. The aforementioned glowing meteor found its way into the water table, and the PCs (and lots of other folks) have been drinking groovy psi-water for "long enough" now. A huge explosion blew up the glowy alien element, now it's particulate matter everyone's breathing in. Fallout, in other words.
Glowy psi-active crystal dust can be from space rocks, or it can be the crystallized blood of the god at the planet's core, that is beginning to seep up and out, to be mined, or whatever.
Milk of the Psi-Poppy. "Everybody does it." "It's in everything here."
The psi-equivalent of gods or greater demons/angels have long been buried in the earth, their minds dormant. But now they're waking up, and granting favors. [snip think I'll keep that one]
The Dark Phoenix, a creature of vast cosmic power, died in a cataclysmic event. Its death scream touched the galaxy, discharging its Force, and imparting many with strange new powers, abilities, visions, and even goals.
A rare cosmological event aligns astronomical bodies, focusing and directing, like a lens, psi-boosting energy toward a certain vaguely Tolkienesque planet...
