It can be a number of things causing this, but I will give you every option I can think of to help out.
Starting just before setting up on the bar.
Hand placement/shoulder and chest flexibility-practice shoulder dislocates with a broom in your house EVERY day. Do about 10 reps. Each rep bring your grip in a little bit more. Over the course of a few months you may be surprised when you can do a dislocate with hands shoulder width apart. This will make it easier to maintain your arch when squatting.
For hand placement it really varies based on preference, type of squat and body type. I was trained by powerlifter to bring my grip in fairly close, but quite so much as like an olympic squat. Never liked it. I got a great arch and a tight back with this grip placement, but since I basically do an athletic full squat and because of my body type it caused issues with me getting down all the way and caused excessive lean. Having seen hundreds of pictures of different squats with some serious weight used I went with a wide thumbless grip. I know most peopel will be like OMG, that's stupid. Not for me. I don't focus so much on gripping the bar as letting the weight of the bar sit in my traps and my hands just kinda balance the bar. If I have to bail out of the squat it's even easier for me because all i have to do is let go. No issue with the thumb trying to hold on. Not saying it's better, but this wide thumbless grip allows me to maintain a good arch, go ass to floor and have a natural feeling body position that allows me to generate my real power coming out of the hole. YMMV. If any doubts are present then do not use a thumbless grip.
Stance and foot width-If you want to do a full squat then my honest opinion is your heels should not be any wider than about 2" past your shoulder. For me shoudler width works fine. Make sure you have a littel toe flare as when you squat all the way down you should end up feelign like you are sitting between your legs, NOT on them. I have made this mistake in the past and when using anything over about 200lbs with toes straight ahead you can go all the way down only to have your hamstrings literally sitting on your calves and if you relax at all this can cause ALOT of tension right above the knee cap and strain that spot. Always have some toe flare, but don't go overkill since you are doing a full squat.
When you descend focus on pushing your ass back to start the descent, but don't go overboard and try to do it like a powerlifter as it can throw your balance off. The important thing is just to break at the hips, not the knees.
Always squat facing away from the mirror. I cannot stress this enough. If you are worried about any form issues have someone experienced monitor your form. When you are squatting in front of a mirror you tend to look at yourself and you descend which number 1 takes your eyes from where they should be which is upwards or straight forward. It also negatively reinforces of depending on the mirror to see how your form is doing. Squat lookign away from the mirror. Pick a spot at eye level or slightly above and keep your eyes locked on that spot. If that causes issues due to your build with maintaining an arch then just keep them neutral looking straight forward as you go down and up. When you initially break at the hips, don't be afraid to let your body naturally incline forward to where it wants to go so long as you keep your back flat or slightly arched as this is liek your balance point between your ass and head.
When you hit rock bottom, regardless of where your eyes are focused for mit helps to look upward a decent amount and focus above all else on pushing my hips forward. If you do this right it feels a little odd as when you pass the sticking point your hips kinda pop forward. Also you may notice that the stress seems to be taken away a certain amount from the legs and transfered to your hips. I read somewhere once that imagine a big ass gorilla is behind you and pulling you shoulders back with one hand and pushing your ass forward with the other. Best I can say is try to push your hips forward as you rise and this will naturally give you a good position. It may be a little harder at first as it does transfer some power away from the legs, but it also increases hip power and thrust over time as it reinforces the exploding of the hips forward which is crucial to any athletic activity from jumping, to sprinting to picking up a heavy weight...
Do some stretching before your squats. Focus on stretching your chest. Do dislocates before if you can because they really stretch the chest hard and loosen up the shoulders. Do soem free weights squats with hands forward for balance and descend all the way down and just sit their like that for a few seconds for the stretch. Try that a couple times. Stretch your calves. Do several toe touches for the hamstrings. Pull your hands back to stretch the biceps and forearms.
That's about everything I can think of. Oh ya, practice squats at home with a broom on your back as well.
Hope this helps.
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