It is possible the fermentation had not completed before you racked to the secondary. At least two specific gravity readings with a hydrometer are needed to confirm final gravity.
The dry hop can also create nucleation points for the CO2 in solution to begin off gassing through the air lock.
Take a gravity reading today and another two days from now. Two days apart for a low gravity beer may be okay especially with the amount of time since the start of active fermentation. If the SG readings are the same begin bottling.
In the future I would not rack to a secondary vessel until final gravity is confirmed. Though I don't use a secondary anymore. I dry hop in the primary after final gravity is confirmed.