Cause:Why it happened is anybody's guess. You could try running the Hack Conflict Detection Utility (
http://www.simwardrobe) and see if you have any conflicts. Also scan your Downloads folder with Clean Installer, sorting by Type, and delete any 32 character filenames with no type (hacks you didn't know you had that were picked up from downloaded lots).
Fix:You could also use SimPE to fix the family relationships to get things back to where they should be. Also get the FFS Lot Debugger from
http://www.moreawesomethanyou.com/smf/ Click through it and select the various fixes, to include Clear Trash from Memories.
If this happened to me, my biggest concern would be is this a symptom of a bigger problem with the neighborhood, and if it's totally corrupted now.