Same thing happens to me, usually on the last rep or two. Technique and supporting muscles are so limiting on the squat, leg strength i should be moving a lot more weight, but it's hard to get everything to come together. I'd say a lot of it is to do with glutes not activating which is a huge problem for me, they are so loose in a squat. Moved 100kg for 2 x 4 today improved on last weeks 100 x 3RM. Been squatting as my lower power exercise for about a month now, might swap it out for deadlifts soon, although i'm never sure if deads actually provide much hypertrophy.