What happened in this kill? Bug or something else?

https://classic.warcraftlogs.com/reports/NrPWY1FnzaLqtXKC#fight=49&type=damage-done

after watching the replay its kinda insane to see baron melting in a blink of an eye. Even their mage has 24 Frostbolt casts on a 12 second bossfight. If someone can explain it, i am happy to get answers!

Cheers!