After taking a single loss (even if it's just in one round), there's a chance that attacks of a certain type can strip away her protection (which significantly reduced incoming damage to her). This leaves her extremely vulnerable to strikes, as her massive, soft belly becomes completely exposed to any and all attacks. She takes nearly double the damage from some heavy blows, and massive damage from combo attacks targeted at her gut plus, certain Brutalities can be unleashed within a specific window of time right after her cuirass flies off and she's left in a momentary daze, making it easy to finish her off quickly in that state.
In the animation, I tried to convey how the straps give way - they can't handle the weight of that thick iron armor - and unexpectedly for Maria, the cuirass comes loose from her bare, sweat - drenched belly, crashing loudly to the floor and causing her to panic in shock.