In my case (and quite a few others, too)
it has been carb cravings (as well as being a chocoholic ).
In some, edema (water retention) has been the culprit.
I haven't heard of any other explanation for the gains.
I don't think it is chemically induced by the med itself
that causes weight gain, just appetite increase for carbohydrates
and, water retention in others.
It is listed as a s/e on the literature, tho.