Causes include sitting quickly and carelessly in hot, humid weather when the twins are just a-danglin', shifting in car seats while seat-belted, any form of self-induced frontal wedgie (often from scooting forward on a cloth-upholstered seat), or a combination of the above.
In particularly blessed gents, sack bunch can result in sitting on one's own balls. Honorable and impressive as the feat sounds, the sensation makes one want to cry and puke simultaneously. Not recommended.
Male Driver: Gaah! I got sack bunch! If you just planted your ass on your own man-marbles, you'd be skittish too!