It wouldn’t prevent the explosive filler from going off from sympathetic detonation – or deflagration in the case of most propellants. A small explosive charge is used to set off TNT, RDX and other shell fillers anyway. Most HE shells have a thin casing for a larger payload.

I cannot say with certainty if the propellant’s deflagration would have the force to set off the filler in the shells, but that appears to be what happened in this case, so…

The propellant contains both the fuel and oxygen needed for it to burn, what it lacks for this reaction is the initial temperature. As the primer goes off it starts a chain reaction and through thermal degradation, the oxidizer inside the propellant mixture starts oxidizing(or otherwise burning) the fuel, one of it’s byproducts being gas which does propel the shell forward and down the barrel. Now as it’s not an ideal reaction, some propellant isn’t burned while still inside the barrel(mainly due to a lack of sufficient oxygen), however when it does meet atmospheric air(i.e. rich in oxygen) it combusts and you get muzzle flash.

Now what else you would need to know is that no detonation actually takes place(but actually a deflagration). A detonation means that a shockwave must be part of the ignition sequence that is propelled through the medium and be part of what makes the medium explode. The propellant itself does NOT explode but rather burns vigorously. In order for an explosion to be…well…considered an explosion, it must take place in a container where no pressure would be allowed to escape(essentially being released all at once in what is to be the actual explosion). Inside a breech and a gun, this does not happen – as the warhead starts moving it creates more and more space for the pressure to escape through – which eventually culminates in the shell leaving the barrel and whatever’s left, behind it.
Now what happens in an ammorack explosion? Basically the propellant starts cooking off, burning. As the tank is buttoned down the pressure rises and rises up to a certain point when a structural weakness gives way and the pressure has a point through which to escape.
That’s the basics of what happens when ammoracks go up.

Now..the HEAT shell DOES use explosives, namely High Explosives as it’s name stands for High Explosive Anti Tank, while HE means High Explosive.

Incidentally AFAIK thermite charges are sometimes used to *wreck* artillery pieces by melting and fusing the breech, so, yeahhhh.