Your center of gravity is shifting sightly throughout the whole squat. In order to keep that weight, that’s high up, balanced throughout the move. You need to engage and tighten your core and lower back to create a pillar of support.
You see some guys squat and they take and hold a breath first. Holding the breath is part of a technique for maintaining a tight core throughout the movement.
It’s not going to build a 6 pack, but it will help maintain a strong core.