It's to do with wing upwash/downwash.
If you think about it, the engine wants a good parallel flow of air into the intake - generally the air in the proximity of the tail will have a small downward vertical component due to the downwash as it leaves the wing. This explains the 'upward' part of your question.
As for the 'slight' upward part, well, the engine is mounted some distance above the wings vertically, so although the air in this region will be affected, it will only be, as you put it, 'slight'.
I also understand the other posters to be correct too, regarding a pitching moment, but I know nothing more about this.
