She cause too much material fatigue on the shoulder straps while she was holding them in that position. Normally they would have snapped in that position when she walked in, but some of the threads were able to make it through that due to their orientation relative to the force of tension being applied. However once she has the swimsuit on properly she started to apply force on the swimsuit by tugging down on it to accentuate her breasts, which caused the few remaining strands to unravel.