While approaching a red light I slow down and shift down to the middle chainring, 3rd cog from the left on the cassette.

Though, when the light turns green and I put some force on the pedals, the chain pops up to the 2nd cog, and the chain drops to the smallest chainring.

What can be the origin of this issue? I really hate it that this happens. It's already cost me a dropout and a set of jockey wheels and I'm not really into getting more things fixed.

Who can enlighten me?