jhaydock answered it correct. If you look at your image, your pivot point is in the center. that will cause 1 side to go up and 1 side to go down. do what jhaydock said

hit insert to pull up your piviot point. Then what i usually do it select the command snap to edge and then move the piviot point until it snaps the the back end of that face u dont want to move up and then hit insert again to get out the pivot menu. i have way grouped together but Jhaydock said the exact same thing so if you follow what he said, you will be fine