Alright, I am not 100% sure it starts losing sources. What I do know that happens is this: when a large file completes downloading, say I wake up and check its progress and I see it's all done, well, at that point it seems that all the other files in the queue (it may be a dozen of them) have 0 sources.
So, it's not like I'm there seeing the sources numbers go down, but I figure it's a little bit too much for *all* the other files to not have *any* sources pretty much every time a big download completes.