Wow, those are very nice.
I've been on a yorkshire pudding kick recently, and while I do have good luck, I havent had them "pop" that much, with that little batter.
The best results I've had are milk & egg, beaten till very fluffy, add flour/salt (all at room temp). That goes into the muffin tins with a smoking hot mixture of oil/pan drippings. I let it go at 400-450 for 20 mins or so.
(this is off the top of my head).
Did you do something "special"? :)