Your muscles are kind of like a rope-and-pulley system, when a muscle contracts it pulls on surrounding muscles which in turn pull on other muscles, ligaments, tendons, etc. that are wrapped around the bone. For example your biceps (when the contract) pull your forearm inward (through connected muscles) and your triceps pull your forearm downwards extending your arm. Squats are so tough on your thighs because your thigh muscles are part of the system that pulls your lower leg closed or causes it to extend. Your muscles NEVER push, only pull.
That bulge you see is a major shift in muscle and body weight that triggers a closing movement or an extension (stretch). Your body has to counter-balance itself, if you throw too much weight to one side you fall over. When one part of your body contracts to stretch another part, another part of the body must also contract to counter the weight caused by the first contracting mass, and so on.
If you stick your butt out then you have to counter its weight with your chest, arms, legs, etc. to keep your balance. When you do so you are creating bulges to counter your butt's bulging mass. It might be your abdomen muscles contracting to pull your torso downwards and folding inward or your lower back muscles pulling your torso straight to stretch it to upright position, etc.
It is a little confusing, but if you really look at images of people you'll see it.
EDIT: The leg is a tricky one but a good example of what's going on.
The upper muscles of the thigh have a kind of X threading and the pull from the top of the thigh on the bottom of the lower leg. The muscles under the thigh actually pull on the muscles on the top of the lower leg, causing it to swing downward. The muscles behind your knee play a role in maneuvering your foot muscles.
The muscles of your upper back work your neck (pulling on the back of it to bring your head up and tilt it back, etc.) while the muscles in your upper chest pull to bring your neck forward and tilt your head down etc. It is all a system of opposing pulling and contracting and counter-contracting.
I'll try to whip up a tutorial of sorts tomorrow to explain it all better, this is all off-the-cuff thinking here from my tired brain which is long due for some sleep.
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