On power down it happens when the voltage goes low the uC enters undefined state (and accidentally modifies content of your battery backuped RAM). You have to ensure a RESET to uC when supply voltage drops (a simple low cost voltage supervisor would be OK - combined with the WDOG timer in one chip like TC1232 or DS1232, LT1232 or similar). When VCC is lower than 5V-5% (10%), it resets a uC and prevents him entering undefined state. I had the same problem with my GOLD-CAP backed up I2C RAM and RTC. In the case of 1 per 100, this had happened. After installing voltage supervisory chips, no problem persists.

Hope this helps.