It was probably the result of a transferred vibration… There isn’t twitching after brain stem activity ceases. If someone got shot in the head, and is twitching, it’s because their brain stem wasn’t damaged. Death reflexes are mostly muscle spasms, and they occur during the final stages of death. There is also something called “tidal breath”. That’s the very last bit of air left in the lungs, and is usually expelled during transportation of the body. The death rattle is the person’s last dying breath, which is different. Lol. Suffice to say… there is absolutely no physical reason (nerves, or otherwise) for the movement you saw. Caskets balance rather precariously on something we call a berm, and transfer energy easily. Even footsteps on a wooden floor can cause trembling inside of the casket. Hope I helped at least a little.