Over-watering. Could also be under-watering, any one or more of several nutritional deficiencies, or natural senescence, but I doubt the latter if it's occurring in numbers sufficient to cause you to ask.

Al

Maybe it is the latter then because it isn't actually happening in large numbers. I was just wondering if the few could be happening for a specific reason. It is on just one plant, my rubber tree plant, and just on a few of the lower leaves. It gets the same watering and minerals as all the rest. I intially thought maybe the lower leaves were not getting any light because of the overhang of the upper leaves. I guess I thought light had something to do with it but maybe it is just time for those lower leaves to go bye bye and make room for more growth. Such a guessing game.

If it's just a few of the lower leaves, and these leaves are rather old, they could just be dying off naturally of "old age"... I usually only worry when it's either newer leaves yellowing and dying, or a lot of leaves yellowing and dying!