Files starting with eZ_REST_BinaryFile are created when uploading a binary (e.g. ezimage field type).
Those amount of files keeps growing and taking up huge amounts of storage.
Especially on Platform.sh this directory is not cleaned when deploying a new version. The files have to be removed manually at the moment.
As soon as the disk is full, the system is no longer usable (binary upload, sessions, etc).
The files are created twice. The upload of one image created those two files in my test: