It's not.
For some reason it seems to happen more when the dragon falls on an uneven surface (basically, when they're sliding or flopping around). I've had two post-1.4 that didn't burn up.
I also wondered if the surface they fall on had something to do with it... perhaps some surfaces cause the physics to think the corpse is still 'moving' so the burn-up doesn't trigger.
Did you have this issue at Shearpoint, by any chance?
Anybody notice that the Light Step perk doesn't save you from all pressure plates? Most of the time it does...and the other times I catch a battering ram/poisoned darts/steel mace in the face parts. And no, I don't mean tripwires. I mean the regular gray stones with the swirly patterns on.
Haven't tried it myself, but I know some pressure plates
need to be triggered to complete a quest or open a quest-related door. Not an excuse in itself, but perhaps it explains the root of the problem.
I can't help but think ALL followers should have the perk, since they're basically useless in dungeons - constantly getting fried by traps or hammered through walls by those stupid pendulums.
On the subject of NPC followers, they also need to fix:
# Follower inventories - specifically self-duplicating items (usually low-end Bows) which gradually over-encumber.
# Items picked up by followers having inverted ownership properties - any 'disowned' item they pick up gets marked as 'stolen', and vice-versa.
# Followers not using their optimal loadout - Aela, for example, still wears leather boots and gloves, and wields an iron bow, even when equipped with full Elven gear.
# Some way to know the skills of a follower would be nice, so we can equip them properly. In the original Fallout, they had a speech option where they simply told you what they felt they were good with.
Some people have a problem in their game and others do not. Konw why? These games ar massed produce by the thousands. All it takes is a small interuption in the game transfer from master to the game disc, and you get a glicth. Most glitches are software, but some are do to manufacture.
CRC checks and disc verification have pretty much eliminated disc printing errors. The downloadable patches wouldn't install onto data from a misprinted disc. I think it's safe to say the problems
are software. DIfferent users have different experiences because everyone plays the game a different way - which is kind of the point. Every open-world Bethesda game has had the same issues at launch.
Again, posting in this thread kind of defeats the purpose if you're not using 1.4 (and you really should be, it improves every aspect of the game with no negative consequences).