What could be the reason for a sudden onset in knee pain, particularly coming up from a squat or sitting on a motorcycle?

Hello Karin, Christine and Harris,
Thank you so much for your answers and your questions. I must clarify that this isn’t really one of my clients. This is my son and, since he lives in another state, I can’t easily assess his form. Yes, he is 30-40 pounds overweight but he hasn’t really added on a lot of exercise recently. The last I heard, the pain is only intermittent now and I will definitely encourage him to see an MD, if the pain persists.
Thank you all again for your answers.