Yeah I never experienced it myself while on tours, but some people can hack the online island. They'll add rocks, trees, whatever (maybe even pwp buildings?) Sometimes it's just to be funny and show off their skills. Other times they're trying to be malicious, so people can't go back to the hut and sell stuff, go on tours, buy stuff, or add their catches to the box.
They can also add patterns to the ground, which isn't malicious either. They can be harmless and cool or petty and lame.