Probably the new plaster. I'm not an expert on plaster pools, but if I remember from reading about the issue, new plaster will cause a significant pH rise for almost a year. Lots of MA at the beginning is just part of the process.
During the first month or so after fresh plaster is applied, you need to test and adjust the PH at least daily, and sometimes more often. Other than that, daily is recommended, but weekly is usually fine.
I've gone an entire pool season without having to adjust the PH once, though usually I do end up needing to adjust the PH once or twice a year. (I have a SWG.)
Our new pool, which was filled with water at the end of January, suffers from the same problem mentioned here, in that the pH keeps rising in it. We have to add about 36oz of muriatic acid roughly every other day to keep it anywhere near 7.4.
I'm told the acid demand will decrease over the first year...and can already see it just beginning to go down a bit.
Well, I've had my pool since August 2010 and I continue to add approximately 1 gallon of muriatic acid about every 10 days to keep my pH in check. I do have an SWG and a waterfall that drops about 1 foot and 1 foot wide. Just a part of doing business. I don't think my plaster is still curing. I don't add anything else regularly, so the cost is minimal.