Mitchell had a Pokemon battle with his older sister in which she defeated his Murkrow fairly easily. He didn't like the fact that he was defeated so easily and got mad at his sister and ran off.
Mitchell began practicing with his Murkrow by throwing things at it to help it deflect things. Unknowingly, he began to throw more and more at his Murkrow until it was unable to deflect the incoming rocks and got hurt. Murkrow got mad and flew off.
Ash and his friends first encountered Murkrow when it created two illusions. First it created a Giratina which almost squished Ash and the second one was a flood which Ash and his friends thought they were going to drown.
Mitchell made up with his Murkrow and asked it to help save his sister and her Misdreavus. It agreed and was able to help defeat Team Rocket.