Why is water collecting in the base of my furnace?
It is indeed condensate water, high efficiency furnaces wring water all year long (more so during heating, but A/C in summer too).

What has happened is the condensate collection point (on the inside of where the external PVC drain runs) has cracked.  It is a very brittle and cheap plastic, just saw this on another customers furnace.  It acts like a little trap, holding water until it overflows into the PVC to drain.  

Possibly the PVC got kicked and the force made its way back to the box (good luck getting a show of hands for this one)..

In any case, your furnace vendor will likely have one in stock.