Velka as the rogue goddess probably doesn't follow the "Chosen Undead" narrative.
I think her position is more likely to be aligned with those Kaathe and the Darkwraiths of New Londo.
Based on what you and others have posted here, i see what you mean. but slightly off-topic: as i said i'm working under the assumption that frampt and kaathe are in it together (many topics about this on various sites if you should need a reference, so i wont get into that topic to deep here), and their manipulation have various parties working together to a single goal sometimes without them even knowing it. From what we know about Velka, she would not be interested in assisting in the chosen undead plot. But the crow from the Asylum might be a contradiction to this.
Some people think the giant crow is Velka, or at least a servant of Velka, and they may be right.
Why does she transport Undead from The Undead Asylum to Lordran? Maybe she does it so that they might one day kill the gods.
This is the easiest and most likely answer, but as i said before, its a boring and uninteresting one
More interesting, and much more speculative, would be that she is willingly or unwillingly assisting the snakes in furthering their goal (1).
another explanation is that she is aware of the plot, and is trying to take advantage of it to fullfil her own interests (2).
Some people have theorized that Velka might be the mother of Gwynns children, i have not researched that angle and do not know if there is even the slightest piece of evidence of that claim, but maybe i should research it to see if that might shed light on her motivations, and her portraitation in the DS games because if that were true, it might clarify scenario (2).
Id like to say again that i am not really looking to contribute to the lore-community with this post, but that im looking for a way to piece velka into my own, very personal interpretation, so forgive me for my heavy speculation that is most likely very un-interesting to the lore-hunters who wanna see evidence for lore-theories.