Bug or Hacker?

I was walking around Pandaria and i saw this guy.

https://gyazo.com/18b0ea3cd77d910fed61dbed97a63f54

No clue if he's hacking or if it's a server bug.

it is a bug, caused by Druid talent.
Some "smart" druid do it on purpose to abuse it (so it is harder to see them), from your screen is impossible to accuse anyone of course

oh ok,
thx :D