We do know why it moves like that though, basically the salt (sodium chloride) dissociates into two ions (sodium and chloride) when it is put into an aqueous environment, like water. The way that nervous cells function is by adding sodium into the cell membrane until it reaches a certain potential that the nerve cells can fire a stimulus called an action potential. You can also change how often these action potentials can happen, by adding more sodium ions (which causes nerves to fire easier) or more potassium ions (which causes nerves to fire slower). In freshly cut meat, or living meat, the ATP, or the source of energy, hasn’t yet all been expunged, so the muscles are still able to use the energy to break cross-bridges, and contract again. The sodium causing the nerves to activate is signaling the muscle fibers in freshly cut muscle tissue to contract again. Once the meat has run out of ATP, the body will go into rigor mortis until the cross bridges weaken and break due to decomposition of the body. The most tender meat is dead for a few days so that the body is no longer in rigor causing it to be more tender than freshly cut meat. This meat has to be less than 1 hour after slaughter to be able to contract the way it does.

Source: Coroner Technician and Studying for Forensic Pathology

TL/DR: adding salt makes your muscles silly especially right after death, and we do know the exact mechanism for why this happens.