Yes, it's surprising how that works, how the hammock can pump the boats rolling and make it roll quite a bit more. Both fun and annoying -- there should be a word for things like that.
I think the root of what's happening is that as the boat heels, the your weight in the hammock is pulling down on it's attachment points to heel the boat more. And this tends to pump the boat to roll more and more. It works so 'well' because the hammock's attachment point is far above the center of rolling, so it's sort of like heeling the boat by attaching a rope
to the mast
and pulling sideways, and since the hammock swings, it ends up pumping the boat and rolling it like someone is rhythmically pushing down on the rail.
So the way to fix this is to either lower the attachment point (to reduce the lever arm), or make the attachment point offset (by attaching it to the side of bow pulpit, or maybe using a whisker pole that's slightly off center).
Ropes didn't work for us -- they just made it jerky and un-hammocky.