Summary: It was only a momentary slip of the beasts’ feet, but it was enough to rock the Ark which was on the cart they were pulling. The priest driving the cart reached back to steady the Ark so it wouldn’t fall; but the moment he touched the Ark, he fell dead.
“David and all the house of Israel were celebrating before the LORD, with songs and lyres and harps and tambourines and castanets and cymbals. And when they came to the threshing floor of Nacon, Uzzah put out his hand to the ark of God and took hold of it, for the oxen stumbled. And the anger of the LORD was kindled against Uzzah, and God struck him down there because of his error, and he died there beside the ark of God. And David was angry because the LORD had broken out against Uzzah. And that place is called Perez-uzzah to this day. And David was afraid of the LORD that day, and he said, ‘How can the ark of the LORD come to me?’ So, David was not willing to take the ark of the LORD into the city of David. But David took it aside to the house of Obed-edom the Gittite. And the ark of the LORD remained in the house of Obed-edom the Gittite three months, and the LORD blessed Obed-edom and all his household.” 
The people were worshipping with genuine enthusiasm—celebrating before the LORD! There was laughter, music, singing, dancing. The congregation was filled with joy—rapturous, untrammeled joy. The dulcimer sounds of lyres and harps harmonised with the mournful sound of trumpets as tambourines and cymbals kept time [cf. 1 CHRONICLES 13:8]. The king was leading the festivities, dancing with all his might, when the oxen stumbled. Over thirty thousand hand-selected men were accompanying the Ark of the Covenant, led by the King himself.
It was only a momentary slip of the beasts’ feet, but it was enough to rock the Ark which was on the cart they were pulling. The priest driving the cart reached back to steady the Ark so it wouldn’t fall; but the moment he touched the Ark, he fell dead. Everything changed in that one moment. Those blowing on the trumpets lowered them in shock at what they witnessed. The ones strumming on the lyres and the harps stopped moving their hands across the strings. The cymbals and tambourines hung loosely from hands as the dancers stopped their lively steps. Everyone stared silently; God had struck Uzzah dead.
WORSHIP THE LORD IN THE BEAUTY OF HOLINESS — “David and all the house of Israel were celebrating before the LORD, with songs and lyres and harps and tambourines and castanets and cymbals.” For seventy years, the Ark of the Covenant had been held by the Philistines. These warlike peoples defeated Israel in battle, capturing the Ark of the Covenant [see 1 SAMUEL 4:1-11]. The Ark was kept as a trophy of war, but the Philistines were most uncomfortable with this symbol of the LORD God in their midst.
Strange things happened wherever the Ark was situated. The Ark was set in the Temple of Dagon, but each morning the idol of Dagon was toppled and found the following morning on the face. At last, the idol suffered the indignity of having the head and hands cut off and placed on the threshold of the Temple. It was unnerving how this kept happening. On top of that, the people suffered with unexplained tumours in the city of Ashdod. So, the Ark was sent to Gath so those people could enjoy the spoils of war [see 1 SAMUEL 5:1-8].