I don't think tornados are actually that uncommon. North America's tornado alley is special, but even Germany has roof-destroying tornados pretty much every year - they just aren't as big or as frequent.

North central Pangea might have been a contender.

Yeah. And far as I understand, the dinosaur fossils we pull out of the great plains are there because that was the shore line of the Western Interior Seaway. I would expect a massive Seaway to mess up whatever conditions create Tornado Alley.

Also, even within Tornado Alley, the prevalence of big tornados is extremely sensitive to localized quirks of geography. Certain places, like Moore, OK, have been hit with F4 and F5 events several times, while apparent safe zones just a few miles away get hit rarely at all.