It’s definitely not the case that cracking your knuckles is the only cause of arthritis. There was a man who cracked his knuckles every day for many many years (until he was about 70 years old) but only on one hand. And he didn’t get arthritis in that hand, or in the other one for that matter! In fact it almost certainly doesn’t have any effect on your chances of getting arthritis
Arthritis is a disorder of the autoimmune system – which means that the body’s immune system for some reason starts attacking itself.