Could be a few things.
You might be bottling before fermentation is 100% complete. This will leave fermentable sugars in the bottle that will contribute to extra carbonation.
You could have a mild infection. Some types of bacteria or wild yeasts can eat some of the sugars beer yeasts can't, this is sometimes called a 'gusher infection'.
I had this problem a while back with a hard apple cider I made. Cracked open a few bottles about 2 weeks after bottling and they were the expected carbonation but needed to age. After 3 months, they were all gushers. Because I didn't have to boil the cider I just heated a little to make sure the extra sugars all dissolved and must have introduced a wild yeast/bacteria due a gap in my sanitary procedure. They seemed to get a touch sour so I assume it was lactobacter sp or similar.