The uniquely male sensation of having one's scrotum squished into an uncomfortably small area or configuration due to unfortunate seating arrangements. The most common culprit is that poorly-placed knot in your blue jeans where all four denim panels get sewn together at Scro Central.
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.
Lady Passenger: Why are you grabbing at yourself? Shouldn't you be concentrating on driving?
Male Driver: Gaah! I got sack bunch! If you just planted your ass on your own man-marbles, you'd be skittish too!