Return Air Vents Icing Up

For the second time this year, the return air vents in our fridge have iced over, preventing the freezer from cooling the fridge properly. Picture is attached.

The first time this happened, we assumed it was caused by our leaving the door open; however, I don't think we have left it open this time. Any advice on what may be causing this and how we could prevent it? It happened in ~3 months this time.