Steam simply stores games in it's install folder, and while downloading the (compressed) game files it keeps them fragmented in a separate directory. As far as I can tell it doesn't employ special low-level APIs, because on lower power hardware (and even sometimes on gaming gear) the bottleneck is often the decompression step. This is what steam is doing when you are downloading a game and it stops using the network but the disk usage is going and the processor gets pinned at 100%.

I also heard of this happening to regular users downloading stuff with safari. It is simply terrible design on apple's part that you can kill a macOS install simply by filling it up so much that it becomes possible to not be able to delete files.