Whats happening is the plant feels it can't or shouldn't support that leaf anymore and is drawing the expensive chlorophyll out of the leaf to re-use somewhere else. Hence the dark green veins where the chlorophyll is concentrating as it ships out, and the yellow lear it leaves behind.

Most likely, this has nothing to do with anything being wrong, its just recycling resources away from lower leaves that aren't getting much light and passing them along to the top leaves that do.

Long as it isn't the top leaves that are discoloring, I wouldn't worry about it, mine do the same thing.

If your leaves are yellow and veins green it's a magnesium deficiency. You can Google it and see images. If its only bottom 1 or 2 leaves would not worry about it as Edymnion stated.