Clouds (like, the big fluffy things in the sky) are made up of many droplets of liquid. Using "droplet" to refer to the things that make up cloud computing is a pretty natural nickname for any cloud provider, not just DO. I do imagine that DO uses "droplet" as a public product branding because it works well with their "Ocean" brand, though.

...now I'm actually interested in knowing if "droplet" is derived from "ocean", or if "Digital Ocean" was derived from having many droplets (which was derived from cloud). Maybe neither.