> Why is that the case [...]?

This is explained in the "Encryption" section: https://borgbackup.readthedocs.io/en/stable/internals/securi...

The important part is the part about avoiding re-use of the AES CTR value.

> Simultaneous updates happen quite often.

Personally I created a dedicated borg repository per machine I want to backup, because that avoids sharing passphrases across machines. This comes with the drawback that I cannot deduplicate across machines, but that is acceptable to me, because the data is mostly unique-ish anyway. I only backup the user data, not everything (e.g. /bin/).