Most commonly when a girl's breasts are so tightly squeezed into a dress/shirt that they are ALMOST falling out. Also if the shirt is too low, or doesn't have enough coverage.
Can also be used if the girl's breasts have actually fallen out of the garment.
Honey, you might want to readjust your dress, you're popping out.
I was so drunk, I was stumbling all over the place, and my shirt was so low I actually popped out of it, and I didn't notice until Karen pointed it out to me!! I was so embarrassed!