And suddenly the whole frag

So my tank is mostly full of leathers, kenya trees, and a few small LPS frags (candy cane and frogspawn), except for one decent sized acropora that I had up until now.  It had been doing great for months under the same condition until just last week it started bleaching and was completely dead in under a week.  I checked my water parameters and the only thing that was off even a little was the dKH which was low by 1 point, so I brought it back up, but everything else seemed just fine.  The funny thing is that this happened right as I was noticing that the coral was starting to grow new branches.

Does anyone have any idea why an acropora would just die like that?  All of my other corals are fine (they were a little stressed while the acro was dying, but now they are all back to normal).

I don't want to give up on SPS corals, and so I'm hoping someone might have a hint as to why this might  happen or maybe this is normal.

A leather coral right next to it retracted all of its polyps as did another leather of a different type that was about half way across the tank from the acro, but in the direct path of a current that was drawn from the area near the acro.  Both of the leathers recovered completely after about 3 days after the acro was completely dead.

I would have fragged the acro if it were not that someone on the forum recommended not to frag a bleaching coral since it could recover and retake the dead parts.  I'm not running carbon right now, but would it help with cases of biological warfare?

Bleaching and RTN look different as well. I have had RTN which you will notice it starts in one spot and rapidly progresses along the coral. If it bleaches it normally does it all in one. I have a purple acro that bleached because of my LED's but you could still see the polyps extend and it is now regaining color. RTN will not regain color once it's gone it's gone.

Even fragging won't guarantee survival. However it will greatly increase your chances. In the event it happens again, make sure you cut well above the effected area. Dipping your cut pieces in iodine will also help fight off further infection. Make sure this is done in a separate container / outside the aquarium.

Cut into the living flesh. I have rarely if ever been able to save the Acro frags from RTN otherwise.

I'm sorry, but it looks pretty certain to me like the many leather coral and other soft coral in the tank ganged up and caused the demise of the Acro. When a leather retracts/shrinks, it let's loose with a whole lotta problem causing chemicals. Acro's and all SPS coral have their own chemicals that bother other coral, including other SPS, but it sounds like in your tank the Acro was way outnumbered by the sheer volume (not number) of leather coral. Leather are the worst for this because they grow fast and soon overtake because of their size or the volume of their population.

The Problem
It's called Alelopathy; the chemical warfare that coral engage in to keep other coral from moving in to their territory. Unfortunately, in our small tanks they are all in the same small territory. The chemical laden water just goes around and around and never leaves. Water changes don't help much. A 10% water change reduces the amount of killing chemicals by only 10%. Larger water changes are a pain, create other problems, and are uneconomical.

The Solution
All reef tanks should run AC(activated carbon) for 2 weeks per month. AC removes a whole lotta chemicals from the water that skimming and algae growth cannot remove. Used to be, before we found out that we should run AC periodically, that there were soft coral tanks and stony coral tanks but mixed tanks didn't do so well. The regular periodic use of AC has changed that.
Just a bridal veil material bag of a handful or two of AC in a high flow area, ideally set over or next to a screened pump intake is all it takes.  Pelletized AC is just as useful as granulated because of it's porosity. It also stays easier inside 50 cents worth of bridal veil material gathered loosely and wrapped together with an elastic at the top. Too tightly packed does not allow water to flow past all the pellets.

Hope this helps.