I used to deadlift like that as well. If you look through my videos you will see the progression I have made. I still have some strides to go.
1. Your hips are too high as Samuel said.
2. The weight is likely not starting over your mid foot.
3. Your back is rounding some.
Here are some tips to get you in a better/safer starting position.
1. Place the bar over the mid foot. This should be a couple of inches from your shins.
2. Go down and grab the bar by bending over at the hip. Your knee angle should not have changed at this point.
3. Move your shins forward until they touch the bar.
4. Expand your chest and tighten up your abs.
5. Experiment with looking down and straight ahead to maintain your arch. I personally look straight ahead.
But you are very strong and if you improve your leverages you will show your stength even more.