As long as you're fighting in an open area it's virtually impossible to
stop pushing from happening. It can, however, be
controlled.
The first step in controlling push in an open area is removing virtually everyone from the back of the mob. Only rogues should be behind the mob, and they should only be angled in from the side. The DPS instead need to be positioned from the sides. Left and right. Spread evenly if possible, but if you can't, that's not the end of the world. You'll have plenty of time to balance push.
The second step is for the warrior to realize what's going to happen. He's going to have to stand still and rotate, instead of strafing to move with the target as it gets pushed to the side. The target will turn to face him, and the DPS will adjust to stay on the sides. In this way, the mob will rotate around the MT in a circle (clockwise or counterclockwise).
However, 9 times out of 10 it's just easier to find a flat wall nearby and pin it, as about 20% of the raid won't understand what's meant to happen and will be hitting from behind out of routine. Given enough time, those 20% will push the mob on top of the MT, which makes the mob flip, and blam - chaos ensues
And er, yeah, Kessdona faces a random player in the raid and does her frontal AE after a delay of 3 seconds or so. The easiest way to handle this is to pin Kess to a corner, keep everyone on one side of her (behind her is usually where we place us) and when she turns everyone moves to the side. Rinse, repeat. If you pick the fighting spot right, it'll allow you to virtually trivialize the encounter.