Bras, especially big bras, have a pretty tough job on their hands... or, erm, cups. They have to deal with both static load and dynamic load, and be both sturdy enough to give support, and flexible enough to allow movement. And breasts are heavy, which in turn means they can generate quite incredible amounts of momentum even during simple, everyday tasks.
Unfortunately for poor Joy here, it appears that this pleasingly sturdy full-cup bra just can't quite take the job on, busting a strap in protest. On the plus-side, at least for us, it gives a rather charming and whimsical slump, as the remaining supported cup sloshes it's load over to the other, and gravity attempts to wrangle its wicked way with the struggling fabric.
As an aside, bra straps generally tend to be pretty strong, especially on larger bras where they contain less elastane or even be similar to a rigid webbing. However, I will say that these straps are looking a little skimpy for the design and size of bra - perhaps another cup size up and the manufacturer would have upgraded to something wider and more substantial?
Hopefully Joy can fix it, or perhaps some kind soul will help her buy a new, slightly larger bra?