Well i am a bit LOST for WORDS bro !
U need to check for any Loose ur cables goin into and out of the UPS if then also everything is fine and even then ur UPS goes into BACK UP mode for no reason, the best way will be to call a technician for the UPS and replace any parts that (MIGHT) have been damaged by any power surge without ur Notice also get check ur wiring of the ROOM in wich u hv kept ur UPS to check there is any sign of any Short Circuit,also try to Operate ur UPS without turning ON ur AC...do post ur results and let others also see this forum to help u better...
cheers n peace...