Albert Barnes Commentary 1 Chronicles 15:26

"And it came to pass, when God helped the Levites that bare the ark of the covenant of Jehovah, that they sacrificed seven bullocks and seven rams." — 1 Chronicles 15:26 (ASV)

When God helped the Levites — The death of Uzza had deeply impressed both David and the Levites, and it was doubted whether God would allow the ark to be moved anymore. Sacrificial animals were held ready; and when the ark moved six paces (2 Samuel 6:13) without any manifestation of divine displeasure, showing that God was not opposing but rather helping the Levites in their task, the victims were immediately offered.