It looks like its heavier than the 50 weight limit that the player and the ground share, which would be why it cant be moved other than between containers.
Since it looks like it weighs less than 65 one good workaround is to transfer the bag/wagon to a nearby container that can hold its weight, breaking the container will let it fall to the floor despite any weight restrictions and you can take items out until the bag/wagon is light enough to pick up.