I'm not sure anyone 100% understands all aspects of burn-in, but I like to think of it as breaking in a pair of jeans, something getting used to a pattern of wear. Semiconductor & capacitor properties vary with temperature, load, rf environment, etc. and modern micro-components also have quantum-scale effects so any number of complex chemical and physical interactions can take place over time. Just because we don't understand something doesn't mean it isn't having real effects! --- Burn-in is a real, common effect, and while it's causes are not known, the number and diversity of factors that affect semiconductor performance mean that physical changes cannot be ruled out. I also believe the human mind helps amplify the effect, like a placebo. Is burn-in real? Is it placebo? I suspect the answer is a mix of both.