This isn't a hard question. There's two reasons pepperonis curl. One is that the top is cooking faster than the bottom, and as with any piece of meat that will cause it to curl towards the surface being cooked faster.
The other reason is that the edge of the pepperoni cooks faster than the middle, that means that the edge experiences tension which causes the pepperoni to fold.
The article addresses both of these factors and introduces an additional important factor your comment doesn't mention. It may be a harder question than you think.
Yet there are pepperonis that don't curl (which some pizzerias allegedly prefer) as discussed in the article. It appears that the way the pepperoni is stuffed, so that the meat forms a U if you take a cross section, gives it a propensity to curl in the first place.
The other reason is that the edge of the pepperoni cooks faster than the middle, that means that the edge experiences tension which causes the pepperoni to fold.