AC condensation line dripping from attic drain and splashing onto drywall
An AC unit will produce condensate all the time, not just when its over a certain temp outside.

Check your AC Unit in the attic.  The Primary AC condensate drain is normally drained to either the main house drain or to the outside via a PVC pipe about the same size as the on draining into the garage. 

There is a secondary drain, think an emergency notification that the Primary Drain is clogged.  or not working.  The drain you are seeing in the garage is most likely hooked to the drip pan hanging under the AC unit to catch any leaks not handled by the main drain, or the AC Air Handler sweating in the heat of the attic.   In most areas the drip pan is supposed to be an electrical float system to shut off the AC unit is there is too much condensate in it, as that means there is something going on with the AC unit.  

Use the attic access in the garage right next the drain pipe and see if that is the drain attached to the Air Handler itself or the emergency drain attached to the drip pan. 

If you see another PVC pipe connected to the Air Handler itself,  follow it in the attic to see where it goes.  If it leads outside, it normally will be draining somewhere near the AC Compressor outside, but might be somewhere else. 
If you find it outside, a quick fix is to clear it out using a wet vac.

Long and short Pics of the AC unit in the attic will help diagnose the issues, but most certainly a call to the AC Service Company is most likely a good thing if they have not been out to Service the AC system this year!