Albert Barnes Commentary 2 Samuel 6:13

"And it was so, that, when they that bare the ark of Jehovah had gone six paces, he sacrificed an ox and a fatling." — 2 Samuel 6:13 (ASV)

The meaning is not that they sacrificed oxen and fattened cattle every six steps, as this would have been impossible. Rather, the sacrifice occurred after an initial, successful journey. Following the arrangements David made for the Levites to carry the ark (1 Chronicles 15:2, 12, 15), they carried it for six paces from the house of Obed-edom. Seeing these visible signs of God's favor, and accompanied by the sound of musical instruments, they then stopped and offered solemn sacrifices.

It is also possible that the word “step” had a technical meaning, referring to a specific distance, such as a stadium. Six such distances would have been nearly a mile. If the terrain was difficult and steep, the successful progress of those who carried the ark over this distance would have been a fitting reason for a thanksgiving sacrifice.