Because they provoked his spirit - literally, “They made his spirit bitter,” or they embittered his soul. They threw him off his guard, so that instead of manifesting the meekness and gentleness which so eminently characterized him in general (see Numbers 12:3), he gave way to expressions of anger. See Numbers 20:10.
So that he spake unadvisedly with his lips - Passionately; in a severe, harsh, and threatening manner. He did not bear with them as he should have done; he did not refer to God, to his power, and to his goodness as he should have done; he spake as if the whole thing depended on him and Aaron: “Hear now, ye rebels; must we fetch you water out of this rock?” The word rendered “spake unadvisedly” - בטא bâṭâ' - means properly to “babble;” and then, to talk idly, or unadvisedly; to utter that which has no meaning, or an improper meaning. Let us not harshly blame Moses, until we are placed in circumstances similar to his, and see how we would ourselves act. Who is there that would not have been provoked as he was, or even to a greater degree? If there are any such, let them “cast the first stone.”