It'll be all the fibre going into your system, I tend to try and drink more water to counter the bloating on green days. I love carbs so it tends to work!
I find that if I have a couple of green days in a row, the first one bloats me, but the second one doesn't so much. Might just be an adjustment period for the carbs after EE and original days where I tend to mainly stick to meat and veg I think!