Just your average cartel power-grab/supply route turf battle. Local report is that the shoot-out was between Gulf Cartel factions and was related to Rio Bravo battles. This is interesting because for last 5-7 years, at least, Rio Bravo has been a Zeta stronghold (Nuevo Progresso is actually considered part of Rio Bravo in that the municipal government in Rio Bravo also oversees Nuevo Progresso). If the Gulf Cartel is operating in Rio Bravo that suggests the Zetas have been run out of town. Nuevo Progresso has by and large managed to avoid the violence that has plagued Reynosa and Matamoros, most likely because of its small size and relative unimportance in not being an active or potential Plaza. From what I have heard, the shoot-out was at the Pemex station on the south side of town by the Autopista. My guess is that the clash occurred there as the differing factions ran into town off the Matamoros-Reynosa Autopista.
http://www.krgv.com/news/nuevo-progreso-residents-deny-reports-of-violence/