You are not getting tight enough at all. You are "hanging from the bar" as opposed to pining it to your back from the beginning - watch the squat grip videos and compare it to what you're doing. Take and hold each of your valsalvs. Bend over early in the squat and get your knees where they should be, and keep them there, until you are halfway back up. Do some TUBOW. Do these things and check back.