The best part is the tourists' response to smelling the burning ligero.
As for combustion, the viso and ligero of some varieties produce an oily finish that enables them to burn quite well.
I'll disagree with the simple explanation that higher leaves are better nourished because they are exposed to more sunlight. Leaf nutrition is regulated by the roots, in order to optimize plant growth toward blossoming (the whole purpose of a plant). Lower leaves are exposed to sunlight for more weeks than upper leaves, but they begin their senescence earlier. Since their maximum growth is supported by immature roots, they end up with less nourishment than upper leaf, and lower nicotine (produced only in the roots, and transported to the leaves).
With a fully mature root system, the plant surges toward blossoming with maximum nutrition. This results in increasingly well nourished leaf toward the upper part of the plant. Topping the plants then logjams the nutrients into those upper leaves.
Bob