(this is mostly speculation and advanced hearsay) Skyrim already has weird problems on the PS3 like severe slowdowns that affect the late-game because the world state and the save file grows too much.

http://venturebeat.com/2011/12/06/fixing-skyrim-would-take-a...

Still, this is not a new problem - Las Vegas had it, Skyrim has it again. Its not like it hit them suddenly. I imagine that Dawnguard basically goes over the top in that regard. Still, it seems like they chose an implementation of a specific problem that does not address the specifics of a major target platform, which sucks.