The conditions for nucleation of abnormal grain growth have been investigated using a 2D vertex simulation allowing for the presence of pinning centers. The topological characteristics of an "almost pinned" structure, compared to the classical grain growth with no pinning force, show a striking correlation between the grain size and the size of the neighbors of this potential abnormal grain. The main finding of this contribution is to stress the crucial importance of pinning particles, and to point to the stochastic nature of the nucleation event in abnormal grain growth.