The objects may fall out of groups if the whole storage system is manually deleted, or the objects themselves seem to disappear (SNMP doesn't report them), or they were manually deleted. When they are re-added (either manually or automatically when the monitor sees them again after some SNMP issue), they will not be put back into the groups they were originally in.
I would check the "audit.log" and see if anyone really deleted the groups.
Also check "dfmmonitor.log" (and rotated copies) for messages about the monitor deleting/adding them (look for the storage system name, if the whole system disappeared).
Note that OM/DFM won't immediately delete anything that disappears, and the FAQ tells you how long it waits, so short disappearances usually don't cause any issue like this.
Cheers,
Jonathan