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,

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.