Mobility & Chest UP!
Without a form check video--which is really the gold standard, putting this text-based feedback to shame--this sounds like a mobility issue combined with not keeping your chest up.
Your back and hips (and ankles, from the sound of your other question) are not flexible enough to get into a deep squat position while maintaining a chest-up position. This is worst in the first few light sets because heavier loads on the bar push your hips deeper, helping you achieve closer to maximum mobility in the hip joint.
Keep squatting, deloading if necessary, and keep your mind blazingly focused on CHEST UP. That means squeezing the shoulderblades together, puffing out your chest like you're on Muscle Beach, and taking (and keeping) a big breath held through the whole rep. If you can't keep these up during all your sets, deload.
If the problem persists, consider switching to high-bar back squats or front squats.