Why are so many teachers leaving?

This begs two immediate questions: what are they doing? And, more importantly, why aren’t they teaching?
As to the first. I’ve no idea. Maybe all teachers leaving employment should be asked to complete an exit questionnaire stating their reasons for leaving and details of what they’re going to do instead – this, I’m sure, would make fascinating reading.
Neither am I sure why they leave. It’s something of a cliché that teaching’s a tough gig: yeah sure, it’s stressful at certain points in the year, and the workload can sometimes seem overwhelming, but is it really that bad? Of course I’ve read some of the horror stories on the TES forums, but are these really representative of most new teachers’ experiences?
Maybe they are: a recent NASUWT survey showed that 84% of teachers felt demoralised and de-professionalised and that over 50% of teachers had seriously considered leaving the profession in the previous 12 months. I’ve heard countless tales of burn out, appalling student behaviour and workplace bullying, but without hard data any solution will be based on the twin devils of hearsay and rumour.