she wasn't suddenly and convieniently. In an earlier issue she was pulled out from between the cracks in reality due to that dream corridor thing, but Brian just assumed she was an illusion. when the thing was shut down she just ended up sort of crashing there.
The thing was caused by a demon so I suppose it's not that impossible that it'd have a connection to hell.
I'm honestly not a big fan of Ellis but I do recall by the end of Excalibur, even if she had gained confidence and control, she still shapeshifted involuntarily. Like when her wedding dress was being made she gave them nightmares because her measurements kept changing.
I'm just afraid the way Cornell's writing her that she won't have any shifting power left, that she's basically locked herself down.