Feel free to reach out to Imperva, ask them if they are part of the transaction flow (like IBM Tealeaf was at a number of my clients before it flipped to Acoustic Experience Analytics), and how a trigger/hit on greylisted IP addresses can cause errors in a transactional system.

Or there's a cache mishit between the front-end web app and back-end inventory system, but this is less likely since Delta (and others) use cached data. It would be too expensive to query the real inventory every time.
Then again, that would throw a friendly error that the seat/inventory is unavailable.

[Edited for content]