it's actually very common, I believe, for variegated plants to try to 'revert' back to green. Less chlorophyll means the plant has to work harder to survive.

I have the variegated abutilon, 'Savitzii', and it's a slow grower and shy bloomer. One day I looked at it and a long branch of totally green leaves had appeared -- literally within a two week period, there were at least four or five leaves on it! I cut it off immediately, or it would have taken over in short order.

I also have a variegated hydrangea that over the winter, lost its leaves, got cut back, and is now growing back almost completely green. It's in competition with a vibernum right next to it and I think if it had remained variegated it would have not been able to 'stand up' to its neighbor!

i once read that if you feed a variegated plant too much it will turn green.