I think it's because it's stored on pallets on its side, and some sugar trickles into the folded paper at the top, ready for you to open it...
Handy tip No. 1 ... get a resealable sugar container ... take it shopping with you ... open the sugar & fill your container in the shop ... get the cashier to scan the empty sugar packet ... all the mess is in the shop, not your kitchen ... job's a good 'un.
Another idea is to place the sugar package in the plastic bag from the grocer's and when you open it, all the excess will fly into the plastic bag. I think place the opened sugar package into a resealable plastic bag.