When you share the calendar, are you allowing the invitees to "edit" or "view only"? If they are able to edit, someone may be accepting the invite and then going back and deciding to delete the calendar from their device. This would delete it for everyone else as well. On the iPhone, there is a toggle to turn on Shared Calendar Alerts under Mail, Contacts, Calendars. If you enable that, you will know if someone changes, deletes, modifies any of the events on the calendar.
I made the mistake of allowing to "edit" and I got one alert after another in rapid-fire succession as someone deleted the events that weren't pertinent to them. I was frantically trying to call them and tell them to stop deleting. I had to re-enter almost 30 different scheduled events that I had luckily written down because when she deleted them, it deleted them across all of my devices simultaneously.
We're facing the same problem with one iPhone 5.
And it's completely crazy.
When enabling the calendar in the settings, it disappears from the calendar list in the iCal app.
When disabled in the settings, the calendar appears in iCal app ! But not the events of course...
Hope Apple will fix this soon !