This isn't a problem so much now, but a batch I made for my father in-law had this problem. A few months back I made a light lager. It sat in the primary for about a month, hit my FG, transfered to a secondary and lagered at high 30s for about a month. I bottled this batch like normal, 5 oz of corn sugar to 5 gallons. The sugar was disolved in boiling water and cooled then added to the bottling bucket at the beginning of the siphone, I did also add 1/4 - 1/2 pack of dry ale yeast to the bucket to make sure everything carbed up well.
I usually try a beer each week into aging to see how it's going. After week one the beer was a bit green and under carbed as expected. Week 2, I got a gusher which I really wasn't ready for. Both beers were from the end of the batch that I knew wouldn't be as clear and I might have gotten some trub so the plan was for them to be test subjects. I did open a second bottle out of the middle of one of cases and the carb level was about where it should be at 2 weeks.
Anyone have an explanation for this? I know it's not a fermentation problem and I doubt it could be an infection. I guess it could be the sugar didn't mix well, but adding it at the start of the transfer, like I always have, I doubt this is it either. If I can't nail this down I doubt I will give the beer to him even if I don't see any other gushers just because I would hate to get that dreaded phone call.