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.
In the couple of times I've been to Club Tortimer online, one of the most common types I've encountered are the arguers. I walk up, say hi, they say hi back, then they keep going with their fierce debate over things usually completely irrelevant to ACNL...and I'm just standing there. Watching.
When I ask if they want to do minigames, they flatly refuse, and don't stop arguing until they feel the issue has been litigated enough. Then they leave.
Like WTF?