Well I'll be straight and say that the hackshield consider now steam and xfire running together is restricted  and as result,  it boot you out from the game.
I am not providing anymore information. This drama belongs to SDE with their ninja hackshield patch.

How do I know it's related to steam and xfire?

Simply, run both process and NF.
Run NF without xfire but with steam.

Results show the obvious.


Oh and the irony, this does not happen to the test server.