Question about 1991 US Proof sets

I was in a coin shop today digging around and I noticed the 1991 proof sets were 3 times as much as others in the same age. They were even more then the 1960 proof set I bought.

What’s up with that? What is so special about this year?

Anyone?

There was a silver dollar issued that year, but I doubt it’s included in the regular proof sets. Maybe just supply vs.demand?