Barbell squats work some muscles more than others. If you do a lot more squats than opposing exercise, you will create imbalanced muscle strength over time.

Of course if you run a careful program, you can make sure to work opposing muscle groups equally and avoid this. But the fact is that a lot of people who lift weights don’t run careful programs.

Another factor is the elasticity of your muscles. If you don’t up your stretching efforts as you up your lifting, you risk trading range of movement for strength. Again—totally manageable but that doesn’t mean everyone does it.