Airlock sucking. Not a temperature issue.

[mysteryshrimp]

This is something I've never seen before. I have a 2 gallon batch of Brett beer in a 3 gallon glass carboy. Fermentation was extremely rapid early, close to finished in 3 days. It's now 3 weeks in, it's been sucking for about 10 days. I have 3 other glass carboys next to it and they are behaving normally. I'm not that concerned about infecting it, as this was an experimental batch of a historic style and I wanted it a little funky anyway. My only question is . . . what's going on in there?

Anyone have any thoughts?

I should add: I've taking several samples, and there are no unexpected off-flavors. Nice brett flavor already.