So, the nice thing about ash is that it's mostly carbon. Carbon in its purest form (not bonded to anything) is either diamond (under high heat+pressure), but more likely some kind of coal. So the visual aspect of it could actually be taken care of.
Now what would actually disassemble you in such a way that you fall apart into ashes? I'll give you an idea that came out of an episode of Fringe and an idea that I came up with as well.
Fringe idea: so the Fringe episode you should read/watch is called Earthling. Without giving too much away, people who disintegrated had lower amounts of radiation in their body than normal. I wonder why... (no spoilers here). At any rate, it caused some people to randomly disintegrate without even noticing - to the best of our knowledge it was completely painless too.
My idea: So this is not going to be as cool-sounding but this does tie-in with the Fringe idea except the Fringe episode doesn't go far enough to actually explain any of the science (or maybe I just can't remember it). At any rate, things could fall apart at a intermolecular level if their IMF forces were disabled. There are 3 of them:
- Hydrogen bonding (the strongest, formed by strong electronegativity differences between elements Fluorine, Oxygen, Hydrogen, and Nitrogen, aka FOHN)
- Van der Waals forces (caused by valence electrons in the p-orbitals of between different compounds, creating momentary dipoles and therefore differences in electronegativity)
- London Dispersion Forces (caused by momentary temporary dipoles - that used to be considered to be a kind of Van der Waals, this is the weakest force).
Now, the material part of a living organism on Earth is typically Carbon, and almost all carbon atoms in our body are bonded to Hydrogen, and most of the Hydrogen is bonded to Oxygen because water. Now, depending on how you want people to fall apart, you can pick and choose which forces you want to disable in a finite space. You want all water to just dismantle itself? Disable H-bonds. Want all non-polar compounds in your body to spill away? Disable London Dispersion forces. Want to get something in the middle? Van der Waals, but this would maybe look a bit messier.
To dismantle a person completely, maybe picking all of the above would work. Carbon in every temperature on Earth is a solid, while Hydrogen and Oxygen are almost always gases (except when they're vapour/water/ice). But one thing this wouldn't handle perfectly is energy - when bonds are broken, energy is released. Less so when it comes to IMFs, but still notable. On one hand, this could explain people turning to piles of black soot, but would also describe the smell created by rarer elements in our body like Nitrogen and Sulfur bonding with Hydrogen and Oxygen, creating funny odors.
Keep in mind, these IMF categories were invented by scientists to help us understand things - the principles that link them are integrated into the fabric our universe. Also, keep in mind that we don't know how to do this in real life - if we found an off-button for this it could have terrible consequences.
Please correct my Chemistry wherever I may be wrong. Hope this helps!
EDIT1: Just realized I forgot to overtly answer the question: no, this will not happen to you in real life. Extreme heat would result in combustion - the person would get cooked in front of you, rather than turn to ash. And the extreme heat would have to be applied over time and in different intensities - one temperature for getting rid of all skin, then muscles/veins, melt and boil all the iron in your blood, then finally the calcium of your bones. It wouldn't be a clean process, but definitely more feasible then disabling a fundamental force of nature.
The processes I described would be more instant, and also completely impossible (thank goodness).
EDIT2: AlexP brought up an important point, and upon doing research, I was wrong. Ash is NOT mostly carbon - it's actually 25-45% Calcium Carbonate (CaCO3, I'm not mistaken) and often Calcium Phosphate (as pointed out by SRM). Soot - a byproduct of carbon combusting - is often released, so it could explain the dark discoloration of the remains of a human. But I was wrong about the carbon part - most carbon involved in combustion turns itself into CO2, CO, CH4, some other gas. That being said, most of this still checks out - eliminating IMFs could have this effect if it was something we knew how to do. That's the point I'd like the focus to be on.