Make sure you lockout at the top (Pull your shoulders back, and thrust hips forward). The top half of the movement you should focus on squeezing your glutes and push your hips into the bar. at the very top retract your shoulders to lock it out.
I could tell from the angle, but it looks like you may have been using a power grip/alternating grip. I recommend using a double overhand grip as your primary grip, and switch to the power grip only after the double overhand fails (typically @ 90-100% 1RM).