What would cause additional calendars to show up on a shared mailbox
ie:"calendar1, calendar2, calendar3, etc."?

We have 6 users connecting to a shared functional mailbox and these
additional calendars are showing up seemingly spontaneously.

Everytime the additional calendars pop up our Exchange admin goes into the
mailbox and deletes them but they almost always come back. Sometimes there
are 12, sometimes 30, it varies.
Under what circumstances would this happen?