That's because as explained you're supposed to know that whenever you make an attack you follow the Make an Attack rules found the PHB. Disintegrate is an attack. So the 3 steps described there is what you follow. Step 2 is where damage resolution happens, wildshape specifically says that you revert during the damage resolution step because any remaining damage after you revert is then applied to your real HP pool. The disintegration itself from the spell happens during step 3 of the making an attack section in the effect phase of making an attack.
Er, what?
I am very confused by this description.
Under Making An Attack, I see three phases: Choose target, determine modifiers, resolve attack. There is no way that damage resolution occurs during Phase 2 of this, since Phase 2 has only to do with determining modifiers to the attack roll.
Can you quote or at least give a more specific citation for the exact rule you're talking about? I'm looking at the top of page 194, and Phase 2 absolutely does not have any damage resolution. Everything is in Phase 3.
Furthermore, Wildshape absolutely does not
specifically say that you revert during "the damage resolution step", because it doesn't refer to such a step at all; it may
imply that, by talking about the overflow damage going into your natural form, but it doesn't
specifically say anything of the sort; if it did, the words "damage resolution step" would be present in that text, and they're not.
The third phase does say "Some attacks cause special effects in addition to or instead of damage", yes. But all the damage application and effects are covered by the same text here.
So far as I can tell, while you are polymorphed, the assumed form's hit points
are your hit points. Consider: "You automatically revert if you fall unconscious, drop to 0 hit points, or die." (Page 66, repeated in the more detailed discussion on page 67.) Note that it says if
you drop to 0 hit points. If you haven't dropped to zero hit points unless your druid form drops to zero hit points, then you never revert in the first place, because that's the condition that would trigger reverting.
It does say that you are not
knocked unconscious if the excess damage doesn't reduce "your normal form" to 0 hit points. But the fact that it says "reduce
your normal form to 0 hit points" as opposed to just "reduce
you to 0 hit points" (emphasis mine) suggests strongly that, yes, if this clause comes into play, you
were in fact reduced to zero hit points.
So the only test we have in disintegrate is "If this damage reduces the target to 0 hit points, it is disintegrated." There is no distinction made between current form and natural form, nor is there anything saying "unless something else causes the target to gain more hit points again". The phases you refer to are not what I see in the "Make An Attack" rule.
So I don't see a basis for concluding that the intention is "clear". Furthermore, there's a parallel to be drawn with
True Polymorph, and I think at least one designer has said that, if you concentrate on a
True Polymorph until it becomes "permanent", it is no longer the case that the target reverts to a previous form at full hit points if dropped to zero hit points. On the other hand, that implies a distinction between permanent and non-permanent shape changes.
You also have to understand that the wildshape form and druid are not 2 different creatures and that the spell does not specifically target the wildshape form. It targets the druid who's life pool is only reduced to 0 hit points, which is what the spell is checking, if the excess damage is enough to bring him to 0.
Except that if the druid was never really reduced to 0 hit points, why did the druid revert?