I racked my IPA into a sanitized carboy onto the hops I'm using for dry hopping, the airlock had been still for about 2 day before the racking. But now its in the secondary the airlock started up again, almost faster than before. What's going on in there? I left it in primary for full week, why would it suddenly start releasing so much CO2 again?
Fermented beer contains somewhere around 0.8 volumes of CO₂. When you rack to secondary, you're certainly causing some (however minimal) agitation of the beer, which will cause some CO₂ to be released. You may also be changing temperature, which might cause some CO₂ to be released. And dry-hopping is going to give tons of nucleation sites for CO₂, which will cause some to be released. Depending on the length of primary, it also simply might not have been finished fermenting, and rousing some yeast up during the racking process might get them going again to finish the beer off.