I have seen this on occasion in various boids including anacondas. I feel you hit it on the head and believe that with anacondas in particular, that they are doing this mostly during shedding events while removing the old skin from their heads. Their spectacle protrudes more than most other snakes, and is more prone to this occurrence. As long as it is a short term phenomenon, I don't see an issue with it. If it occurs frequently or lasts for extended periods, it could be a dehydration issue.

Kelly