Because they provoked his spirit, so that he spake unadvisedly with his lips.

Matthew Henry's Concise Commentary

106:13-33 Those that will not wait for God's counsel, shall justly be given up to their own hearts' lusts, to walk in their own counsels. An undue desire, even for lawful things, becomes sinful. God showed his displeasure for this. He filled them with uneasiness of mind, terror of conscience, and self-reproach. Many that fare deliciously every day, and whose bodies are healthful, have leanness in their souls: no love to God, no thankfulness, no appetite for the Bread of life, and then the soul must be lean. Those wretchedly forget themselves, that feast their bodies and starve their souls. Even the true believer will see abundant cause to say, It is of the Lord's mercies that I am not consumed. Often have we set up idols in our hearts, cleaved to some forbidden object; so that if a greater than Moses had not stood to turn away the anger of the Lord, we should have been destroyed. If God dealt severely with Moses for unadvised words, what do those deserve who speak many proud and wicked words? It is just in God to remove those relations that are blessings to us, when we are peevish and provoking to them, and grieve their spirits.