What is causing the leaves to fall off?

I've had this plant here for about two years. In the past few weeks a bunch of leaves have started to change colors and fall off. I haven't changed anything in the routine.

It is near a northwest facing window. Lots of indirect sunlight throughout the day. I water it roughly once a month in the winter and add fertilizer about once a season if I remember. I wipe dust off the leaves once every couple of months or so.

I don't do much more than than that. It's been happy up until now.