I hit all 20 dogs with one arcane explosion, and it crit on every one of them. So 20 total crits.
So it would be the same as hitting one dog 20 seperate times, and having it crit each time? which is (1/5)^20 chance of that happening with 20% crit right?
No, it wouldn't be the same
Theoretically you can crit one target 20/20, though it's highly unlikely not to say impossible. 20% does not mean you'll crit 1/5 either, it just means that you have a chance of critting 1/5, but the dice could lane so you crit 1/10 followed by 2-3 crits followed by no crits for the next 5-10 hits.
This is almost definitely what happened. If there were any death knights in your raid, there's a good chance they were frozen from hungering cold, or possibly even a frost mage casting freeze. Which would be not uncommon on a pull like that, frost mages use freeze on packs to proc fingers of frost, and that would give you your bigger crit.
Yeah, most likely there was some factor that highly increased your chance to crit. BUT, if the value were truly 20%, with 20 crits out of 20 attempts, that's amazing. There's probably a higher chance your addon or combat UI fucked up than actually having that rng.