Because they provoked his spirit, so that he spake unadvisedly with his lips. They provoked his spirit - המרו himru, from מרה marah, to rebel: they brought it into a rebellious state; he was soured and irritated, and was off his guard.
So that he spake unadvisedly with his lips - For this sentence we have only these two words in the Hebrew, ויבטא בשפתיו vayebatte bisephathaiv, he stuttered or stammered with his lips, indicating that he was transported with anger. See the notes on Numbers 20:10-12 (note).