What Yooper says and this: as time progresses lots of chemical changes take place in the mead. Some of those changes cause particles to drop out of suspension (could be proteins, tannins, even yeast, and the like). Those particles act as nucleation sites for the CO2 and enable the gas to collect. As the gas collects it has enough energy now to leave the liquid and when you open the bottle and release the pressure it rockets out of the bottle. In other words, there is no need for fermentation to have continued after you bottled for you to have observed a gusher. If you did not degas before bottling this can happen. (And I am ignoring changes in ambient temperature (warmer liquids cannot keep as much gas dissolved as colder liquids - which is why it can be helpful to increase the temperature of your wine when you are degassing) and/or air pressure.