Airlock drawing back at the end of fermentation

[stan.distortion]

Anyone know why this is? I pretty much always get it at the end of my brews and was wondering what causes it. Judging by the airlock there always negative pressure in the fermentation vessel (water always higher on the vessel side than the open to air side), I've never caught it bubbling backwards but wouldn't be surprised if it happens from time to time.

EDIT. I'll usually raise the temperature by a few degrees after fermentation stops and the airlock is still suggesting it's lower than atmospheric pressure in there, it's in a brewfridge so there would never be a significant drop in temperature.

 

[QED]

What kind of FV do you use? Is it a bucket style? I have seen this with FV's like buckets that balloon (especially the lid) during fermentation and when fermentation becomes less vigorous or stops, the FV goes back to it's normal size and can 'suck' on the airlock.
I've never seen it happen with a rigid FV (like the Better Bottle carboys I use or my recent Fermzilla)
...just my guess!

 

[stan.distortion]

Yes, the cheapo 30L buckets and the lid does bulge up during fermentation. That could make sense, I'd wondered if it was something that happened during diacetyl rest or CO2 absorption, something along those lines but if it doesn't happen with more solid types of vessel then it must be something to do with the bucket.

 

[peebee]

… I've never seen it happen with a rigid FV (like the Better Bottle carboys I use or my recent Fermzilla)
...just my guess!

It happens! Both my fermenters are stainless steel which should be fairly rigid. And I see it happen. Even without changes of temperature (which can produce lower pressures if the change is fairly big; e.g. "cold-crashing", and cause CO2 to be absorbed if temperature drops, but this is countered a bit by the airspace being diluted with air and changing the potential levels of CO2 "saturation" - phah, that's all a bit involved), air pressure does change and a fermenter with an airlock will start acting like a crude barometer; "breathing" in and out to reflect the atmospheric pressure at the time.

People are known to attach bags of CO2 to counter this (some airlocks even have vacuum sensitive by-pass valves to which you can connect them). Left unchecked the airspace above the beer will become the same composition as air, but the beer will have been kegged, casked or bottled long before this.

 

[QED]

It happens!

As well as happening on FV’s that can balloon a bit, I wonder if it can be more prevalent the bigger the lid of your FV is? I quite honestly have never seen it happen when I’m using my carboys. I’ve only recently got my Fermzilla but again, the lid is quite small.
I’m completely hypothesising here so happy to be shot down in flames!