Your pelvis is attached quite firmly to your spine. Whatever your pelvis does, your spine follows. What's happening when you see the dreaded "butt wink" during a squat or deadlift, is the pelvis rotating posteriorly. Your spine follows suit, going into flexion and putting compressive pressure onto the anterior part of the disc, vertebrae and facet joints. Now add whatever weight you are squatting/deadlifting and those forces grow exponentially. This is the most common mechanism for disc herniations.
You can fix this by working on the flexibility and joint mobility in the hips and ankles as well as spending some time on the actual movement mechanics.
Videos of some techniques to mobilize the hip and ankle coming soon.