At its most base description one who has Sprouted holds a Soul which bears far too many Contracts which favor another’s Soul. Enough pull and the Soul begins to naturally protect itself by attempting to siphon a more equal portion of Meyda from Contracts in which it is favored. This begins to drain the Soul rapidly and that divide which draws the line between the Extramaterial World and the Material World thins for the Soul. Meyda leaks from it into the entity which bears it and their body often undergoes rapid symptoms of Meyda-bombardment until it undoes itself Soul and all in a disgusting meltdown of biological matter and Hyper Meyda Density.
The entire construction of the three name system in Shelush was invented and maintained to prevent Sprouting and it has worked for centuries quite effectively. Other hazards in the world exist that can cause such reactions from the Soul and body such as Where-it-all-will-go, but while they bear the same name to the folk of the planet they are not identical phenomena.