1 -> 2, he's unconscious, no explosive.
2 -> 3, forced by the Time Lord council to regenerate, the sound effects are explosive sounds.
3 -> 4, radiation poisoning but He spent 10 years lost in the Time Vortex. he's unconscious, no explosive.
4 -> 5, he FELL. No explosions.
5 -> 6, poisoned. No explosives.
6 -> 7, unknown, likely blunt force trauma, no explosives.
7 -> 8, shot by a bullet, then really killed by an inept surgeon, a very mundane way to go. Unconscious, No explosive regeneration.
8 -> War, reluctant suicide by poison and forced manipulation of the regeneration by non time lord science.
War -> 9, old age. Can't explain why it looked like the new regenerations, likely due to the time stream being out of sync.
9 -> 10, he just absorbed the essence of the TARDIS from Rose, which had made her a physical god.
10 -> 10, he was shot by a Dalek beam, and recovered by channelling the regeneration energy into his conveniently located severed hand.
10 -> 11, he had just absorbed an absurd amount of powerful radiation from the master's machine. And he was fighting it.
River had no experience or training with safe regenerating.
Note 11 into 12 actual transformation, Clara was at the Doctor's finger tips, and the transformation was literally just a flash of light. It was only when he was infused with an entire new set of regeneration energy that he was dangerous, an hour before.
And remember, the Doctor isn't that skilled of a time lord. Many others could choose what they wanted to look like, like Romana and the Master. Even then, the Doctor and River easily pass regeneration energy between them without issue.
So in universe it can be explained as a consequence of how the regeneration gets triggered. If there is any competing energy in his body, it makes the regeneration more explosive.
Out of universe explanation is simple, people are now used to things going boom.