Sometimes it's just the way it goes - you're more tired than you realised, or are dehydrated or underfed etc.
Sometimes it's because one of the muscles in the movement group is a weakness, and as soon as it becomes fatigued the effect on your performance is exponential. For something like the squat you could look at your core work and see whether you are putting in enough time to supporting movements (back extensions, rollouts etc). When I see someone fail a squat, it is usually not due to a lack of leg strength - typically they can't maintain the structural integrity to move the weight through an optimal path.
Sometimes it's because one of the muscles in the movement group is a weakness, and as soon as it becomes fatigued the effect on your performance is exponential. For something like the squat you could look at your core work and see whether you are putting in enough time to supporting movements (back extensions, rollouts etc). When I see someone fail a squat, it is usually not due to a lack of leg strength - typically they can't maintain the structural integrity to move the weight through an optimal path.