When research is at an early stage, it's usually a good idea not to bring along too many "armchair theories" about the creature being studied. At this time, as best as I can tell from a quick trawl of the relevant literature, very little is known about environmental impacts on salamander brains. Impacts can be not merely gross-scale physical insults, but can include chemical insults (for example). So there may be potentially several subtle environmental factors that give rise to the salamander's ability to regenerate parts (I wonder which parts, exactly?) of its brain under certain conditions. Let's just say that this is a fascinating area of research and we're seeing only the earliest steps forward today.