As the name suggests, it is what is always known about Kaalika, The one whose Asana is among the body less spirits.
Kaalika being the Sakthi of the smashana, is surrounded by Spirits, the spirits find comfort and stability in her wake.
She seats herself with in the spirit forces, as if it were an Asana, or seat. This is also one of the reasons why disrespect or casual Sadhana of Kali is not recommended to the unclean as the spirits react very fast to the sadhak in the path of Maa Kali.
The truly accomplished Sadhaka of Ma Kali also finds his asana among the Pretas.
Hence the name, PretaAsana