It was also a regeneration that occurred outside or not near the tardis. I also think it depends on the circumstances of the regenerations.
The first doctor succombed to old age and regenerated in the tardis.
The second doctor had his regeneration forced upon him in a controlled environment with a little memory wiping by the timelords.
The third doctor occurred next to the tardis, resulting in the fourth doctor having a little paranoia and dementia near the beginning.
The fourth doctor regenerated into the fifth under unusual circumstances. First there was the whole tardis within a tardis thing that caused the future doctor (in the form of the watcher) appear, foreshadowing the fourth doctor's impending death. The fourth doctor fell from an extreme height and probably had massive body damage. This coupled with being so far from the Tardis resulted in a rather difficult regeneration.
The fifth into sixth was cause by the spectrox poison, which aparantly was some nasty stuff as it almost caused the doctor to die rather than regenerate (he said it feels differently this time because the poison was perhaps almost killing faster than he could regenerate, a concept presented in the turn left episode of the 4th series). In either sense, the sixth doctor's abbrasive personality resulted from this.
This sixth doctor into seventh was in the tardis, caused by the rani, perhaps a forced regeneration to make the doctor more susceptible to the rani's tricks.
seventh to eight, hindered by the anasthesia. Resulted in massive memory loss.
8th to 9th: who knows.
9th to 10th: Inside the Tardis, but the massive damage done to him caused him to be pretty much unconsious for a whole day.
10th to 11th, remains to be seen.
