Yea, the Radiation + holding it in/resisting the change, was what I always thought.^ I'd thought it was all that Vinvocci radiation, plus the extra time he "held it in".
I think that Ten's regeneration was explosive because 'he didn't want to go.' He fought tooth and nail to stay in his (then) current incarnation.
Also, because it was cool and the new control room is much nicer.![]()