The state of Nuevo Leon, unlike Chihuahua, Baja California, Durango or Veracruz is usually pretty good at reporting known homicides, including massacres and known mass graves, so while the data is not 100% trustworthy it can be considered to reflect homicide counts as publicly known. Though that still leaves the issue that the Mexican government so far has been unable to identify a single one of the victims of the Cadereyta massacre which would imply that there may be more mass graves we just don't know about.
Looking at the data broken down by municipality violence was going up in Cadereyta before the massacre occured. At the AGEB level there is a somewhat inverse relationship between population density and homicides, so while populous municipalities certainly have more homicides, that does not wholly explain differences in number of homicides.
|Source: Estadística de Procuración de Justicia|