Where did I go wrong with this pie?
Simple: it's undercooked. 

So what was the specific variable that caused this outcome? Who knows. Maybe 400 on your oven isn't the same as 400 on the oven of the person who wrote the recipe, or your pie started out with colder ingredients, or you opened the oven door too many times, or any number of other factors which could affect how long something takes to cook in an oven.

More important than where you went wrong, is how you can avoid doing this again in the future. As I sit here typing, I can hear my pastry instructor in school yelling "Put that back in the oven! Golden brown means brown, not gold!" 

You should always cook a pie to the correct color/doneness, not to a specified time. In most cases in cooking, time is the least reliable indicator for doneness. To someone with experience, even smell by itself is more accurate than time.

Good luck with your future pie baking!