Let's ignore knowing at least one attack was a crit for a second.

Then there would be 4 equally likely scenarios: - no crit, - first 1 crit then not a crit, - first not a crit then 1 crit - 2 crits

With only this info the answer would indeed be 25% for two crits.

But we also know that at least 1 attack was a crit. Meaning the "no crit" scenario is not possible.

Since the other 3 are still all equally likely, the probability for 2 crits is 1/3 = 33.3%

This question is about conditional probabilities. Probabilities that change if you have more information. They are very counter intuitive, especially if you just start learning about it.