Yo Micky.
Thought process you ask?
Hmmm,I'm not sure I have one of those.
Usually I just do stuff!
I use fade out when I think that's the way the song ought to go. I'm a fan of a build up towards the end,then fade it out and the party goes on (copyright Boydie)
I think (with me anyway) it's all what the song demands.
One thing I do dislike and you'll find me comment on it frequently,is an inconclusive ending. All that effort goes into writing and recording a fine song then....it either randomly peters out or abruptly stops.
I don't class a fade out as a cop out,like Pauly says,but I can see how it could be used that way,as if the writer cba finishing the song. It's just another tool which works well in context.
I often put more work into ending a song than any other part,because I think beginnings and endings are often a 'do rightly'.