I thought it was a conscious decision to regenerate and that's why the Doctor can in fact be killed before his final regeneration has been reached.
When death comes to quickly and unexpected is over.
That helps with the movie death where he is aware of being shot albeit surprising, triggers the regeneration which has to do its mojo on a clinically dead body , and him commenting with "nearly didn't regenerate" or omething to that effect.
Similarly Left Turn where some UNIT soldiers talk about him not having time to regenerate
and the grace shot of the Dalek weapon an episode later which was just dumb luck. A direct hit and no regeneration either.
When death comes to quickly and unexpected is over.
That helps with the movie death where he is aware of being shot albeit surprising, triggers the regeneration which has to do its mojo on a clinically dead body , and him commenting with "nearly didn't regenerate" or omething to that effect.
Similarly Left Turn where some UNIT soldiers talk about him not having time to regenerate
and the grace shot of the Dalek weapon an episode later which was just dumb luck. A direct hit and no regeneration either.