I believe you already checked, but you may take a look at the obvious first. Does that happened when returning from your loooong trip? Have you added weight or carried more stuff going back home? Is the tire pressure correct? Are the tires worn? If no is the answer to all, then the rear shock needs attention.
1989 Honda XL600V Transalp, slightly modified!
"If you don't follow your dreams, you might as well be a vegetable", Burt Munro, The World's Fastest Indian