From what I've read of it from various sources, nobody really knows what the whole "berserk" thing originally meant. One theory I've seen went so that it came from "bare chest", in the sense of a warrior putting aside his shield (hence "baring" his chest) and grasping his sword with two hands to deliver more powerful blows - an obviously risky and reckless tactic.
¯\(°_o)/¯
Go figure.