Printer triggers my UPS power supply and shuts down my PC

I have a Brother Laser Printer HL-2240D. I also have a back up supply which is a back-UPS XS 900 from APC. It used to work just fine before but lately, when I started it, the lights in my room will flicker. The UPS does not cut the power on my desktop. Yesterday, when I tried to print some documents, my printer suddenly faltered my UPS box and my computer shuts down. I don't know why it suddenly failed. Please help me fix this.
