Why is my neutered dog air humping? Is it a seizure or behavior?

It's hard to say just based on reading a description. "Air humping" is something that many dogs do when they are excited (thats any form of excitement, it's not always sexual in nature). Dogs can also learn behavior by watching, following or being exposed to it from other dogs.
Pets can have focal seizures and unfortunately, they can look like anything. One key though to tell the difference is that when a pet is having a seizure, their mentation is generally alerted. Meaning you call Walter's name and he seems significantly less responsive or out of it. It's because the brain is firing out of sync. When in doubt, I always advise clients to record the episode and also try to get the pet's attention while the episode is occurring. It helps to vet a lot to be able to actually see things happening and watch the response. I hope this helps and please feel free to post any additional questions.