The same thing can also occur if you pitch your yeast while your wort is still a little warm (e.g., 70-80 F), seal up the plastic lid on the bucket, attached the filled 3 piece airlock, and placed it in a fermenation fridge set to 64 F. If you watched it for a while (or came back a few hours later), you'd see that most of the liquid in the airlock had been sucked out of the airlock and back into the fermentor. Hopefully that liquid was nothing more than pre-boiled water or straight vodka.
Similar effects can occur from handling the sealed plastic buckets w/o removing the airlocks--as when transfering from fridge to countertop. For this reason, I always remove the airlock and put a small plastic kiddie cup over the airlock opening during temp changes or handling/transfer steps.