Why is the second condensation drain on my central air unit active all of a sudden?
My FIL is an HVAC guy and he explained it to me when we installed a unit in a house I built.

The first drip line is the main one. Mine had a float switch that would turn off the unit in case it was clogged. 
The 2nd one is a backup. In the case that the float switch fails, a clog in the drain line won't flood your attic (where the unit was installed). On mine we ran it so that it would drip in front of a window, increasing the likelihood I would notice it. 
Sounds like you have a clog in the drain line and yours either doesn't have a switch, or it does and it failed, sending the condensation through the 2nd line.