Archaeologists cannot say why the horse was beheaded, but they assume that it was part of the funeral ceremony.

The human remains belonged to the owner of the horse; the burial was made about 1,400 years ago and belongs to the era of the Merovingian dynasty (A.D. 476–750), whose rule extended from central Europe to modern France, according to the Live Science.
"He stood in a 'chain of command' with the Merovingian kings on its top, which meant he was obliged to participate in the king's campaigns," Folke Damminger, an archaeologist in charge of research at the site, said as quoted by the Live Science.
Archaeologists do not know why the horse was buried beheaded, but perhaps it was part of the funeral ceremony, and the horse was placed nearby as "grave goods" and not as a sacrifice. It is reported that the head of the animal was not found near the burial site.
Next to the man, the remains of seven more people who lived at the same time period were found, among them a woman buried with a golden brooch.