What causes roaming versus regular arthritis?

RA plus others. Migratory arthritis is not uncommon in early RA prior to the onset of chronic disease. Gout under the right circumstances can be migratory moving from joint to joint. Certain infectious forms of arthritis may be migratory, such as gonorrhea, and viral disease such as parvovirus b19. Psoriatic arthritis can be migratory in the chronic since involving different joints at different times.
Many things. Migratory arthritis has several possible causes. If you have true joint swelling that comes and goes you need to get this checked out. Other causes include infection, gout, rheumatoid arthritis among other causes.