When the breasts cause some degree of wardrobe malfunction due to the relatively small size of the clothing in comparison to her larger breast size. Or the breasts look like they will burst or "pop" at any moment.
Examples include when the subject is wearing a blouse so small that her breasts cause her blouse to be pushed open between the buttons, or where buttons are popping off or have previously popped off. This could also be used for breasts suddenly and unexpectedly spilling over the top of tight clothing, much to her surprise, delight, dismay, etc.
This tag should not be used when the garment malfunctions due to some other means, such as the classic bikini top becoming untied or suddenly slipping off.