Why does some mayonnaise burn when toasted? I've been toasting sandwiches with mayo on the outside, and Sir Kensington's Classic Mayonnaise leaves burnt/black stain on the toaster.
Other mayo (kraft...) leaves no black/burnt residue.

Sunflower Oil, Egg Yolks, Water, Lemon Juice, Salt, White Vinegar, Raw Sugar, Ground Mustard Seed, Black Pepper, Citric Acid.

Which ingredient is burning?
With the exception of the water and salt, any of those ingredients will burn if you heat them hot enough. If you're seeing it more with a particular brand, though, and if it's happening at temperatures below 200 degrees Celsius, it's probably the egg. Mass-produced commercial mayonnaise like Kraft has relatively little egg, while 'boutique' mayo (or homemade mayo) will have more.