My theory ....
I fully expect one of the lumberjacks to give a confession at the end of their life that this was a hoax, whether intentional or not. My thought is that they were taking some sort of hallucinogen, maybe shrooms they found in the woods. The drug(s) were stronger than they anticipated, and they had a surreal experience that led to Travis wandering off for a few days without food, water, or adequate clothing, extremely confused, which culminated in him being found naked miles away (but not too many miles for him to walk). I know it is very common for people under the influence of LSD to remove their clothes, I would assume it would be the same for shrooms. I also think some or all of them may have "believed" their experience, or been confused enough to go along with what another logger said. Lie detector tests cannot actually differentiate between truth and lies, they merely measure stress (inconclusive results indicate that no "credible" baseline was established, which could easily be due to this confusion). This is why lie detectors aren't admissible in court. Maybe the pressure for an explanation (or realizing they'd left Travis behind) made them stick to their story.
I guess it's even possible that there was a fungus of some sort in the woods that they were exposed to when cutting down older trees, and they weren't initially aware that they were hallucinating. Unfortunately, since this seemed to have happened in BFE, no one was given a toxicology screening or examined in any way (other than Walter? Sounds like they treated for exposure/psychiatric symptoms but they did not test him).
They're coming to get you, Barbara!