1) PAN candidates crack down more on the actual smuggling of drugs, which forces gangs into more violent ways of earning a living (i.e. kidnapping, bank robbery, et cetera).2) PAN candidates invite new gangs into their entities, which sparks fights with the incumbent criminal groups. (A version of this is the common though not necessarily correct explanation for the spike in violence in Torreón.)
What else could there be? The extent to which independent factors were controlled for would seem to be a big remaining question. Nothing against the student, but so much of what determines the meanders of Mexican security are shadowy variables that are very difficult if not impossible to accurately account for, which makes you hesitate to trust the veracity of any broad conclusion (see previous post). Nonetheless, very, very interesting.