Why does the contents of a partition get copied to a directory named mount point?

Question

Debian

Good time everyone!
Encountered a strange failure on the local server: all the mount points mentioned in fstab were suddenly unmounted , and with strange changes: a directory appeared at the root of each unmounted partition, the name of which coincided with the name of the mount point from fstab ( a subdirectory appeared in the partition allocated under /boot boot , etc.). The contents of the sections turned out to be transferred to these subdirectories (attached to them in the file system). Of course, all the links flew off, the server crashed. After restoring the directory structure from the chroot, booting and running went without problems. But I can’t reproduce the error or understand the reason (at the time of the failure, nothing was running on the server, full Idle state) yet.
Has anyone experienced or knows what is the cause?