Here's the El Universal account of the event:
The night was carrying on normally, dozens of adolescents congregating in houses numbered 2063 and 2069 on the streets Arquitectos and Félix Candela to celebrate a birthday.The party was interrupted suddenly by a man of roughly 20 years who made his way to the center of the people, and shouted who was the owner of a car that was parked outside.Evidently, no one answered him, and he asked again. After not hearing an answer, he lost patience. From his clothes he pulled out a gun and began firing indiscriminately.
It's a little unclear what the exact sequence of events was, but the article refers to two mass killings and multiple gunmen ("brats between 16 and 21 years old", according to witnesses), so I imagine the above account was just the initial episode, followed by further shootings by more people at the other house. No word on whether or not the victims had anything to do with organized crime, but I expect that after Calderón's faux pas following the January massacre, no one will be tossing that accusation around unless they are certain this time. In any event, the article leaves the impression that they were not.
Who knows what the real reason was (mistaken identity, revenge, personal feud, some combination of the three, et cetera), but there doesn't seem to be much of a business motive to this killing. Stories like this, though unusual, support the idea that the violence in Juárez goes well beyond Chapo versus Carrillo.