Many people fail heavy squats by falling forward. Typically, their legs are strong enough to lift the weight but their back muscles are not strong enough to support the weight. Use exercises that strengthen your erector spinae muscle group to overcome falling forward!
The Study: Myer, G. D., Kushner, A. M., Brent, J. L., Schoenfeld, B. J., Hugentobler, J., Lloyd, R. S., … & McGill, S. M. (2014). The back squat: A proposed assessment of functional deficits and technical factors that limit performance. Strength and Conditioning Journal, 36(6), 4.