i think that uriel started all this for a long time now and the angels that died in the "battlefield" were the ones that refused to help lucifer rise again

The subversive angels have probably been causing casualties and blaming the demons- that's why Castiel thought that demons were killing the angels.
OR when he said dying- maybe he meant they were sent back to heaven? What exactly happens when an angel is returned to heaven? They can't just pop back down to earth, can they?

huh. dude! you think such thinky thoughts :]] I don't know man .. I hadn't even really thought about it.

Ok so I'm thinking a combination of a few things.
Maybe they had turned angels, the ones who said yes to Uriel, killing other angels.
Then demons sending angels back to Heaven, like Alister tried to do to Cas. Like ff said its not gonna be like they can just swoop back down to Earth straight away!
Maybe angels are being captured and held hostage by demons and the angels are counting then as lost.
Perhaps if an angel falls others count that as dead.
I'm not sure and now all this conflicting information has confused my poor little brain :(

i think that uriel has been up to this a lot longer than we think...so the ones that said yes were fighting against the other angels and well a few died and they blaimed it on the demons

...but now that I have, it's hard to say. I think Uriel would have killed on the spot any angels that said no to his plan, so they couldn't warn other angels (who may have banded together to stop him), or warn his superiors about what he was up to. But if the demons don't know how to kill the angels.....hmmmm!