Imagine this. You see a new lifter try to max out on the bench press. He lowers the bar all shaky and unstable, bounces it off his chest, gets it halfway up, and stalls. He squirms for a second under the motionless bar and then suddenly drops the weight on himself.
Proprioceptors, like muscle spindles, monitor change in muscle length and tension. When these proprioceptors process tension that could potentially damage tendons and other soft tissues, the Golgi Tendon Organ (GTO) inhibits (stops) muscle action and you end up going limp. The same thing happens to the guy who drops to his knees trying to PR on the squat.
What's Going On?
All this info is being processed by the spinal cord in something called a "feedback loop." The problem is that this occurs well before the untrained lifter gets a chance to exhibit his true maximal strength. He could lift more if he was conditioned to reduce this inhibition to a degree.
Successful lifters intuitively know that to get stronger they have to "learn to grind." What they're doing is learning to ignore this signal so they can endure more tension before their receptors send a signal to shut down the activity.
How to Fight it
While most trainers will say lifting maximal weights is the solution, there's another one that'll work too: flexing maximally against a submaximal weight – lots of muscle tension, relatively light weight.
This is what an experienced lifter is doing when he treats every set the same and pretends like he's lifting 700 pounds when he's warming up with 135 pounds. He's tensing his muscles as much as he can and ignoring what his receptors are telling him (that there's no real weight to be concerned with). He learns to ignore the feedback loop.
This will translate over to heavy lifting. When he lifts maximal weights, he'll be able to grind out that heavy rep.