This is about the oldest
CSM picture I could find amongst my stuff with that backpack, this guy would be from the 80s.
In the old fluff (and I think partly in the new fluff) the warp caused
CSM armour to become merged with their bodies, so that it was itself alive and would bleed when cut. It would also grow and warp which was how they ended up with all the horns and spikes and creepy little faces on their armour. I always got the impression from the old
CSM backpacks that the 'wing style' was definitely caused by exposure to the warp, as it's very organic looking.
The wing style probably wasn't the only way in which
CSM backpacks could warp... But it looked cool, and for practical reasons I guess it became standard because all
CSMs used the same plastic backpack.
Then in 96(ish?)
GW overhauled the whole chaos range when they brought out the first Codex Chaos, and made a new backpack that was more tech looking than warped. Maybe this was deliberate back peddling on
GWs part to explain why all
CSM backpacks had warped in exactly the same way, and imply that they hadn't 'warped' at all... that is just the preherasy style. Which sounds better than "we couldn't be bothered to sculpt a unique warped backpack for every marine" (understandable though that is). I still find it slightly odd that despite the huge variety of heavily warped and twisted Chaos Space Marines... They still all have the same backpacks. As though for some reason backpacks are the only thing completely impervious to the effects of the warp, nurgle's rot, chaos gifts etc...
In any case I much prefer the old organic looking 'warped' backpacks to the more recent ones.
There's just an acre of you fellas, isn't there? 
