Where most collective nouns on this list describe snakes at rest, “slither” is about motion. It’s less a term for a fixed group and more a description of snakes on the move together, whether that’s several individuals migrating toward a den, converging on a food source, or simply crossing the same stretch of open ground at the same time.
The word also just borrows the most recognizable verb in the snake vocabulary. Slithering, technically a form of lateral undulation, is how most snake species get around, pushing sideways against small irregularities in the ground to generate forward motion without legs. Different species have evolved variations on this, sidewinding across loose desert sand, or concertina movement through tight burrows, but the basic wave-like motion is instantly recognizable, and watching several snakes do it at once is presumably exactly what inspired the name.
Unlike “den” or “nest,” “slither” doesn’t point to any specific biological behavior, it’s simply the most snake-like word available.
Example: “A slither of snakes crossed the trail single file just as the sun dropped behind the ridge.”
Related: Collective Nouns for Reptiles · What Is a Group of Snakes Called?