My Time Machine deletes ALL previous backups

Every so often, Time Machine deletes ALL previous backups and starts over from scratch. It has done this several times in the last year. I have not changed any hardware or the name of the computer. Why is this happening? Makes me very concerned that I now have NO backups while it chugs away rebuilding a 210 GB backup.

Just got an error message:

Time Machine couldn’t complete the backup. Time machine could not delete the backup disk image "/Volumes/Joe's iMac.purgeable"

HELP!

iMac 27: 8GB, 2.9 GHz Intel Core I5

OS X Yosemite 10.10.1

Backup volume is Western Digital MyBookLiveDuo with 2TB and 1.8TB available