[MENTION=132373]Convict[/MENTION] can answer, but in short:
1. Everyone hated Michael Clarke.
2. Dave Warner and Michael Clarke hated Shane Watson
3. Shane Watson and Mitch Johnson were close friends.
If you go back and recall the Test series between Australia and Sri Lanka in 2012-13, it becomes sickeningly obvious that Mitchell Johnson hadn't "lost it" at all.
At Melbourne in the First innings he took 14-2-63-4 but in the second innings he took 8-0-16-2 and broke Sangakkara's hand too.
The fact of the matter is that the skipper (Clarke) didn't like Shane Watson's friend. That's why both were then marginalised and ostracised in India, culminating in Homeworkgate.