You could probably create such a species by taking inspiration from several different reptiles and other animals.
Blindworms look deceptively like snakes, but they actually are legless lizzards. Due to differences in evolution, it's plausible they would have eating habits that differ from that of snakes.
It's rare, but not unheard of snakes piling together in earth pits to preserve warmth in areas where the nights can become dangerously cold. Have them evolve in a habitat where small prey gradually went extinct over several generations and you have a plausible reason for the snake-pile seaking out carcasses as a pack.
To create a truely multi-headed snake-pile that moves as one into the direction of the next carcass, you could improve their ability to detect smallest traces of the smell of decay. Vultures are known to accumulate around carcasses despite having only limited social behavior, simply because all birds in a certain area follow the same scent trail. If your snake-pack sleeps in one place, it's plausible that multiple members perceive the scent trail of the next carcass and other members simply follow because this behavior usually yields a meal.
Several species of fish and birds developed movement patterns that keep shoals and flocks together and moving as one unit. The snake-pack doesn't even need very sophisticated swarm-behavior, because they can glide over each other without causing them to crash or being eaten. All they need is a disposition to avoid the direction where none of their kin can be seen and viola! instant snake swarm.
Next mix in some Aligator.
Alligators consume food that cannot be eaten in one bite by allowing it to rot, or by biting and then spinning or convulsing wildly until bite-sized chunks are torn off. This is referred to as a "death roll".
You could easily envision a writhering, convulsing mass of snake-like bodies ripping tiny chunks out of a carcass.
The only thing I cannot quite envision is the snake-pack hunting down live animals. It's far too dangerous for non-poisenous reptiles to hunt prey bigger than them. It would require a tremendous combined effort of the pack that probably
necessitate a level of intelligence far above what reptiles are capable of. Poisenous snakes, on the other hand, have no reason to hunt together.