Magicka degenerating randomly?

Barlthump

So I'm a stam blade and I was running vet maelstorm arena on Friday night. While in the course of getting owned by some whirling blades and what nots I started to realise that my mana bar was randomly going down for no reason. It would just degenerate and cause me to lose my magicka which in turn causes me to not use my double take as intended. Is this really supposed to happen or is it some sort of arena mechanic?