If it happens again, check for a slipped tendon right away. You hold the hock (knee) between thumb and forefinger, and gently roll, like you’re rolling a pencil between your fingers. If the tendon slipped it will snap back into place. Slipping tendons, and most other leg deformities usually stem from improper nutrition. Chicks that slip tendons are often indicative of parents feed being lacking some necessary elements. This is why your older birds may seem fine, and the next gen are ill. The original stock may have had parents with better nutrition available. What exact brands and types do you feed the chicks and adults?