It's likely because they are empty, and thus light. And he probably went and tried to hold it like it was heavy... Well, some bend the body as one holds the thing so that the weight counter balances that, but if there's no weight you are just sort of... Bending yourself backwards.
It's a weirdly terrible sensation. It should be anything much but... Well, I'd compare the feeling with trying to go up one more stair, but there's no stair [or worse, thinking the stairs downward is over but there's one more].
NoEmpty boxEmpty box...This isCan I help you?!Master, that's an amazing amount of luggage?!Wha-?!Ah, no...these are actuall--Snake shippingAllow mine self to take this.Where did you pop out of?!*Ka-CRICK*Hup!GRAMPS!!WHY?!Even though they're just empty boxes, why did your back go out?!