1. Fuel fire - in diesel tanks, hard to ignite the diesel and it burns slowly. Gas/petrol tanks will burn more easily, may leading to an explosion if there is a good fuel-air mix in the fuel tanks.
2. Internal fire cooking off ammo 1: that Panther shows the jets of flame related to the propellant in AP rounds cooking off. While there is plenty of it, propellant burns - it doesn't explode. It is designed to burn the whole period of time that the projectile is travelling up the gun barrel, to provide roughly constant acceleration during the interior ballistics section. One round may burn, igniting others around it - burns very hot and very lethal, may generate enough sudden pressure to pop hatches but not much more.
3. Internal fire cooking off ammo 2: MG ammo ignites and starts to cook off - "firing" MG rounds all over the place, both internally and externally.
4. Internal fire cooking off ammo 3: HE projectiles cook off. This is the HE in the warheads, not the propellant. Called "high explosive" as it goes off as near-instantly as possible, for maximum instant damage. When this happens you see the tank erupt, turret fly, etc. I've seen film of the turret flying off the tank, the turret roof separating and flying etc. The most catastrophic destruction.
None of these things are "instant". In some cases they can be pretty damn quick - high-velocity projectile melting and punching super-heated metal directly into contact with live rounds, or a HEAT plasma jet doing the same. Mostly it requires a fire to start in the wrong place - in an ammo bin or in very close proximity. Many ammo bins were at least partly protected from fire as a safey measure.
And will these explosions be modelled? Yup.
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