There will always be a slight forward lean, especially as the weight increases but once the movement begins there should be no change in trunk angle (the lean should not increae as you go down)
If you can't maintain trunk angle and your butt sticks out like
Donald Duck - then you have a weakness somewhere - without assessing your form and doing some tests its impossible to tell where ?- most common is the deep abdomnal wall & lower segments of the rectus abdominus or the erectors in your low back as well as tight muscles pulling you into that position - anterior pelvic tilt.
One way to check is to stand side ways to a mirror and look at your belt line or top of shorts, if the back is higher than the front, you are heading for trouble! you want the belt line to be horizontal I.e no tilt in your pelvis
now granted there are 2 main styles of squats - powerlifting / bodbuilding. The first emphasizes glutes & hams, the second quads
Each have their own distinct style - depends what you want to do? but to do either with a forward tilt of the pelvis is really bad for your back. You must determine are you just leaning forward due to the heavy weight, bad form or tilted pelvis? all 3 are different and need different corrections
Heavy good mornings are great for the low back, but need to be done as a seperate movement from squats - Actually they kick ass as a movement, only real lifters still use these today, most of the pussies use the back extension machine - total piece of shit!
***please note I am not talking about the reverse hyper bench, back raise bench or GHD bench - I mean the plate loaded girlie machine.
But to have the spine in safe postion and have the hams,glutes erectors and lats work correclty as a force couple you need to do the movements seperately (and correctly)
Later!