Simply because hot air risesThe fan in your oven works to circulate the hot air in the oven, cooking the food efficiently. Naturally, the hot air will want to reach the top of the oven and beyond. This is why, when you open the oven door there is a short, 2 second or so, delay before hot air rushes out from the top of your oven to the ceiling. While this is totally normal, it is something to bear in mind when opening the door to remove food. Keep your hands and face away from the oven cavity for just a few seconds after opening to give the hot air time to escape safely into the room, and not straight at you.