I'm just curious the 12th (13th) Doctor regeneration was explained. Does anyone have their own theory on how the susunod regeneration will be explained?

The explanation was ibingiay in the most kamakailan special episode, called The araw Of The Doctor (I think. All of these "...of the Doctor" titles can get me muddled). He had run out of regenerations and was dying. Clara spoke through a slit in time and puwang to Gallifrey asking the other timelords to help him. They sent him spare regenerations. Most fans are assuming he's got another full set now.