Weak hips. If anything, I'd say abductors, as opposed to adductors, because you have to be able to spread your legs in a low hip position and hold them there, not pull them in. I can see where you would need adductor flexibility though.
i meant abductors, i am a moron.....the muscles you would use to spread your thighs apart....you can use a band doubled around your knees or you can use the machine at the gym especially for these....I usually do them standing on the 3-way hip machine, or I use a sled and drag sideways
Too much weight. lower the weights. You are there to build muscle not show off. That is a classic example of too much weight. If you lighten up and use a weight that allows you to use proper form then your muscles will strengthen in the proper ratio. You can emphasize the area you feel are weak with other excersizes, however, like I said, that is a classic example of using too much weight in the squat causing your form to suffer.
This is a common postural distortion and usually requires correct stretching in addition to strengthening.
Do static stretches of your gastrocs, soleus, and adductors. Stretch until you find slight tension and hold for 20 seconds.
For abductors, try tube-walking. It involes looping a rubber tube around both feet and walking with a wide stance. This will help strengthen the abductors while working synergistically with the rest of your leg in a stabilization role.
Do all of this both before and after you lift to promote the correct movement patterns. You have to teach your CNS to relearn the movement recruitment pattern.