A single Cast on Crit ColdSnap proc depletes full 10k manapool

As title said. I was trying Frostbolt CoC ColdSnap yesterday. While mapping it works normally, but when i fight vs boss, a single trigger of coldsnap on crit on boss brings my mana down from 10k to pretty much 0 - and i was not using archmage. Here's the video - boss fight started around 3:10 mark
https://www.youtube.com/watch?v=YUfLW99YJUE