Elephants are not immune to having health issues. This one appears to have a hurt left rear leg. How it happened you can't say for sure and you can't tell for how long this one has had the problem.
Elephants are social animals to the point where their empathy with each other is so great the herd will stay together to tend to any sick family member. This one would shuffle along on the bad leg and had a partner that stayed right next to it in case it needed help. The main family group would go along it's way but never got too far ahead without stopping to wait until this one would catch up.