One problem - we couldn't find the answer! All our favorite sources from Michael Ruhlman's Ratio to Shirley O. Corriher's BakeWise agreed that it's crucial to keep the oven door closed during baking, but none of them explained why.
The best we got after more poking around in our books and on the web is that popovers won't "pop" properly if you open the door. This makes a little sense - the only leavening in popovers is the steam created as the water evaporates, which then lifts the light batter. This depends on high heat, particularly at the beginning of baking.
If you open the oven door during that time and let out some of the heat, it would make sense that the popovers wouldn't rise as dramatically. As we know, just a few seconds with the door open will dramatically drop the oven temperature.
So maybe it's only important to leave the door closed during the first 10 minutes of baking, but it's ok once the structure is set and the inside is drying out? Maybe it's time we do our own experiments!
What do you think? Anyone have a better answer?
Related: Recipe: Easy Ethereal Popovers