When an army of Daleks were planning to destroy the Earth, the Doctor prepared a delta wave which would have destroyed the Dalek army, but also the Earth as well ("Coward. Any day.") However, Rose Tyler, having looked into the heart of the TARDIS and absorbed the whole of the Time Vortex, destroyed the Dalek army herself. However, it was killing her, and the Doctor absorbed himself by kissing her, saving her but resulting in his regeneration.
Time Lords regenerate because they have a injury or something is wrong, it's their way of healing themselves.
Trenzalore (his final resting place), in the town of Christmas
The eighth Doctor regenerated offscreen and nothing is known about how or why he regenerated. The eighth Doctor appeared onscreen only once in a TV movie. We know th Time War happened between the TV movie and the start of the 2005 series. It's widely assumed that whatever the Doctor did to end the Time War caused him to regenerate.
He regenerated because his current body was getting old, so it was of natural causes.
He died of old age, pretty much. He was an elderly man to begin with, and the stress and strain of defeating the Cybermen in 'The Tenth Planet' took its toll. He collapsed in the TARDIS and in a scene that no video footage remains for, he regenerates into the Second Doctor.
If you are referring to the ninth Doctor in Doctor Who, then the answer is Christopher Eccleston.
Ninth Doctor was created in 2005.
He will regenerate into the 12th Doctor.
He'll regenerate into the 13th Doctor.
There is no episode with the eighth Doctor regenerating, he was only in the 1996 movie, and when the show returned in 2005 it was with the ninth Doctor.
The fourth Doctor fell off a bridge.
Eventually yes, he will regenerate into the twelfth Doctor.
Romana was another Time Lord, like the Doctor.
Peter Davison.
In the movie, The End Of Time.
Time Lords regenerate because they have a injury or something is wrong, it's their way of healing themselves.
Christopher Eccleston