William Hartnell - Old age and general weariness pushed over the edge by stopping the Cybermen.
Patrick Troughton - Forced to regenerate by the Time Lords.
Jon Pertwee - Killed by radiation.
Tom Baker - Fall from a radio tower.
Peter Davison - Poisoned by a toxic substance.
Colin Baker - Err... hit his head on the TARDIS console.
Sylvester McCoy - Shot by some stupid gang member then butchered by the doctors at the hospital.
I take it I can stop there?
