Because Berserk is a skill, and like all skills there is a chance for a dramatic failure. One of the benefits of Berserk is the ability to ignore some damage, possible all at higher levels. When that skill fails dramatically then you do not gain that benefit. The only thing I can tell that controls that is your level in the skill and luck.

It's like fumbling when you get a dramatic failure on a weapon skill or casting an attack spell on yourself on a dramatic spellcasting failure...

, Pantherro's voice