It's a gas...any ideas?

[Wynott]

Last November, I made up a gallon of Elderberry with 1lb Runner Bean liquor (yes, Berry recipe) which had been in the freezer. I thought it had finished fermenting, so put it in the garage to mature (grey owl). Checking in September, it appeared to have started again, so brought it back in. I went to do the final (3rd) racking today, and as well as dropping from 1.000 to 0.992 since 19 October, it was so gassy that I had to stop siphoning 3 times during the process. Now 0.992 is too dry for my taste, so I have put just a little syrup in to bring it up, as it's still going, but can't work out why it has gone so very gassy of a sudden.

Ideas anyone?