There are several factors that can cause this, including improper mixing, using a pan that's not cast iron, letting your pan cool down too much before adding your batter, or baking in an oven that hasn't fully reached temperature. Also, it's important to note that it is completely normal for a Dutch baby to rise and then fall once it comes out of the oven! Just like with popovers, the crazy rise that you see comes from the steam, and as it cools the steam escapes and things start to deflate. Totally normal and still totally delicious!