This isn't really known right now to any significant degree. As opposed to some other parts of Peruvian prehistory, we know relatively little about the Chimú. They had been conquered by the Inka shortly before the Spanish arrived, and some of the Chronicles mention organized displacement of Chimú populations in the Inka mitmaq system. In order to get better ideas about Chimú sacrifice, we have to look to analogy and comparison. In Peru, that brings up two options.
- The first is the Inka system of sacrificing children at ritually significant points of the calendar year. Children were selected in a rigorous process and paraded into and out of the capitol of Cuzco, where they were feasted and celebrated. Often a male and female pair were sacrificed in the same event. Most of our well preserved examples, such as the Peruvian Ice Maiden, come from high altitude sites where the sacrificial victims were well preserved. https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/peruvian-ice-maiden This could very well be what's happening at Huanchaco, just on a larger scale. Supposedly (reported by the highly shocked catholic Spaniards) some elites in the empire viewed having their children selected as an honor.
- Andrushko, V., Buzon, M., Gabaja, A., Mcewan, G., Simonetti, A., & Creaser, R. (2010, September 3). Investigating a child sacrifice event from the Inca heartland. Retrieved from https://www3.nd.edu/~asimonet/Publications/Andrushko_et_al_2010_JAS.pdf
- The other option is that the sacrificial ceremony is descended from Moche rituals which occurred in almost exactly the same place 500-1000 years earlier. In these rituals, defeated warriors were elaborately sacrificed in public spectacles, often after having been tortured. Initial thinking was that these warriors were selected from the local elite, but biodistance work by John Verano and others at Huaca de la Luna suggests that the sacrificed warriors were of non-local, possibly subjugated origin.
- Sutter, R. C., & Verano, J. W. (2006, November 28). Biodistance analysis of the Moche sacrificial victims from Huaca de la Luna plaza 3C: Matrix method test of their origins - Sutter - 2007 - American Journal of Physical Anthropology - Wiley Online Library. Retrieved from https://onlinelibrary.wiley.com/doi/abs/10.1002/ajpa.20514 SutterVeranoAJPA2007.pdf
The big answer to the question is going to come after more analysis is done on the remains to try and determine exactly where they came from.
Sorry for the poorly formatted citations, I'm not great at using reddit's text editor.