When do they happen? is it just kinda random?

i dont know i have helped
and gl with your question

creators of the game "nabi" simply re-stock them any time they wish. I do think that the people who are allowed to are: Gman80, veb, hampa, and any others who may work for nabi.

skulfuk can restock items too

gman80 doesn't work for Nabi anymore.

As it has been said, people with the power to restock ToriShop does it whenever they want.
However, this is really rare. Don't expect it to happen every week or month.