i thought that as well on first watch, the bag seems to jump or suddenly change.

I think, after watching it frame by frame, the bad is being quickly grabbed by the "hands" and over (what looks like, to my untrained eye) 2 frames the bag gets squashed a little.

i think the hand motion is hidden by the arm, the bag is being held in a slightly weird way which makes it change shape unexpectedly and the hand closes very fast (hard?) making the shape change quickly.