Shrugging shoulders mean a lot of things.
1. "WOAH!" (Character can also be leaning back for more emotion)
2. "So?"
3. "I dunno."
4. "AHHHH!" (Surprised and leaning back like 1, except with more fear.)
5. Something in a dance move?
6. Somebody told them to shrug? XD
7. Shoulder exercises?
8. No reason. They're just shrugging without even noticing it.
9. A special gesture or habit.