I'd say it almost definitely is legs, with weak glutes.
Rather than the legs extending because they're strong, it's more a case of the hips shooting back allowing the stronger back to take over.
Glutes as your legs are caving in somewhat on the ascent, in combination with the above.
Stronger legs and glutes would help. Having said that, I do like the squatty bounce style of Squatting it just needs a little more precision. Regular paused Squats focusing on getting deep and opening the knees out on the way up should go some towards alleviating issues. Particularly getting those paused Squats within 10% of your max Squat.