Sweepy gets stuck in unloading status when it has encountered/swept up an amount that the dock is somehow unable to receive.
Happens by chance if sweepy encounters micro amounts of anything. The dock cannot receive sweepy's contents.
In this case it has somehow acquired mcg amounts of isoresin. Has also happened with ice. A molecular forge is nearby.
Destroying sweepy's dock will force liberation of sweepy's contents for relocation and combining with more acceptable mass for future use.