Airlock sucking back in

Ok so I made a pumpkin ale and put it in my conical to ferment on Labor day.Fermentation was great(constant bubbling 24 hours after pitching). After a week or so it settles down like it should. Yesterday I notice the airlock is sucking back into the conical. What causes this to happen?

Pressure inside the fermenter being lower than outside. Probably a temperature thing, the air inside is contracting because it's cooling down.

It happens to me when I have a sudden temperature difference. I ferment in water baths supplemented with ice and ice packs (it sure is hot down here) in a cooler. Whenever I drop the water bath's temp with some ice and there's no fermentation activity taking place, my blowoff starts sucking in the sanitizer. Not much, but enough to notice.

Have not had any intake of liquid yet, but that could happen any day now with this Tripel.