How can I prevent egg casserole from deflating?
What you're making is basically a savory Dutch baby or popover. Not to discourage you, but it's going to be really hard to come up with any way to make them consistently stay puffed; pretty much all egg-puff baked items deflate after puffing.

One easy thing you can do is to use the same step one takes with cream puffs, and prick them with a skewer right after they come out of the oven.  This releases steam from the center; the cooling moisture is actually what causes rapid deflation. You also want the interiors of the puffs to be as dry as possible.

The other things to do are experiment with adding slightly more bisquick, and cooking them slightly longer.  The additional flour should make them "sturdier", but you don't want to add so much they don't puff properly.  Cooking them longer is aimed at drying out the interior.

Of course, if the centers of your puffs are moist and you like them that way, you're going to just have to live with them deflating.  Also, realize that, even after you achieve the best stays-puffed recipe, sometimes (due to weather or timing or oven wonkiness) they'll still collapse on you.