We cover key mechanisms, including sudden changes in tissue loading, deconditioning, and age-related tendon and joint vulnerability (Achilles tendinopathy, rotator cuff tendinopathy/tears, and osteoarthritis flare). We also outline practical clinical red flags and decision points for when patients should seek professional assessment: persistent or escalating pain, night pain, latent symptom patterns after activity, swelling, and—critically—sudden pain with loss of function suggesting possible tendon injury.