https://versusarthritis.org/about-arthritis/conditions/reactive-arthritis/#:~:text=One%20theory%20is%20that%20once,good%20days%20and%20bad%20days.
I don't think anyone really knows what causes reactive arthritis (ReA)

When I was initially diagnosed a doctor said there were three possibilities for my future with ReA.

1) ReA could be a one-time event and never happen again.

2) ReA could have a "recurring pattern" of flares

3) Worst case -- ReA could become chronic and never go away.

For me, it seemed like a one-time event until after 5 years I had another bout of ReA. Then my flares started to recur every year.

After 20 years of recurring flares of ReA, I developed another autoimmune condition that was chronic. My rheumatologist insisted that the other condition wasn't reactive arthritis. Eventually everything was called "systemic inflammation" because now I have been diagnosed with multiple autoimmune disorders.
https://www.autoimmuneinstitute.org/articles/comorbidities-in-autoimmune-disease-multiple-autoimmune-syndrome/