When warm wort cools it sucks. Literally. All you did was learn that suck back occurs because, as the air in the headspace above the wort cools, the air contracts. The contracting air pulls the liquid backwards through the lock. The same effect will happen when crash-cooling beer post-fermentation, if using a liquid air-lock.
I chill as much as possible then drain to a fermenter and cover the blowoff port with foil. Once to pitching temp, I pitch yeast and hook up the blow off tube. When I remove the tube I always remove the blowoff jug side before the fermenter side which seems to reduce issues.
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