[Knufen]

I have a master degree in chemistry and during my studies have grown quite a lot of crystals. As a lot of the other posters point out it is due to capillary effects; both formed by the new crystals and by microscopic channels and irregularities in the container.

If crystals didn't 'want' to grow we would often cool the solution down and scratch the bottom of the glass container with a metal spatula.