Based on the somewhat sketchy description of what happened to the Trackers I'm going to speculate that the cause was direct sunlight entering the binocular. The sunlight could be focused by either the objective or eyepiece lenses into a tiny hot image of the sun falling on what is probably a plastic eyepiece field stop. This could happen from sunlight entering either end, but the eyepiece is more likely just because it accepts light from a much wider angle than the objective. The ambient air temperature wouldn't be much of a factor if a focused image of the sun is doing the damage. A few unlucky circumstances would have to come together for the this to happen. First the angle of the sunlight entering the binocular would have to be just right for the sun's image to fall on the field stop edge and the binocular would have to remain motionless long enough for the plastic to melt. I imagine most really cheap binoculars use plastic field stops, so if this is really the cause, it might happen with any of them.
Good theory, Henry. It sounds reasonable to me. Sort of like a magnifying glass focusing it's beam creating a hot spot. That's what it looked like a plastic field stop melting because it changed and got worse. It almost looked like something dripping or melting. No more real cheap binoculars for me. Even though some of them produce a good image there build quality is cheap and they are not going to last as long as a better constructed higher quality binocular. So in the long run cheap binoculars are actually false economy. Exactly where is the field stop? Can I look at the Nikon Prostaff and tell if it's field stops are plastic or metal? Does anybody that has a Nikon Prostaff know if it's field stops are made of. You know another thing I think the Olympus Lenses are actually made of plastic. If light became focused on them why couldn't they melt?