Think about how often you bend your knee throughout the day…up/down stairs, curbs, squat to pick something up, bend down to tie or shoe, sit down & get up from a chair or a dozen other reasons that require bending your knee. Beyond just bending your knee, you have to control the weight of your body during this raising and lowering. Now, think of how often you may do this in an entire day, week, or month. Over time, you could really be aggravating your knees. That’s not even walking, running, jumping or higher level demands. I say all of this to point out how often you bend your knee. So, if you are moving poorly, you could be overstraining the knee. I am attaching a video to show you how you can assess the quality of knee bend. It is called the lateral step down test. It is pretty straightforward...if you can't step down from a 4-6 inch box without 1) letting your knee cave inward 2) dropping your pelvis or 3) putting your other leg down for balance/stability than you likely have some sort of limitation.
You could have some hip or leg weakness, ankle restrictions, calf/quad tightness, knee pathology, or some other discrepancy. Also, if this causes pain you may want to see previous posts where I have referenced knee pain. It's impossible to determine the underlying cause by one test you have seen on the internet; however, if you can't complete this with the correct form or it causes discomfort you may want to see a healthcare professional.