Always keep at least two straps in your jockey box. And don't count on having the two straps you've just purchased to be returned to you as you may not be live unloaded.
Loads that may need your attention are usually thinly wrapped near the top, thus being more subject to motion than the thicker plastic wrapping around the middle or near the bottom of a pallet.
Imagine fifty pound dog food bags sitting on top of a pallet with only a thin amount of plastic wrapping keeping it in place. If you had to apply the brakes to stop from rolling backwards on an incline, those bags will likely shift.