I think it's that DA2 is very decisive game some love it others hate it but I think it all stems from a vocal group of posters to bash everything bioware related.
Admittedly picking it for a top 5 within a specific pool of games is a lot easier to rationalize than Game of the Year among all choices. Not that I'm saying the choice is wrong of course, but I'd be curious to read the rationale (if there is one), especially given the runner up choices. Guess I have to wait for the link to work again or something first.