The timing is what's so suspect.. Trump could have waited. He could've put this on the GOP leadership in Congress to ask for Comey's resignation.
This came out of left field and Sessions manufactured the reasoning over the past week.
Why not let the process unfold in a way that would not cast such suspicion on the President if Congress was initially behind the move?
The President (or his advisers at a minimum), surely understands, what it means for him to fire the man investigating him. That's why he insulated himself by saying that Comey told him on 3 separate occasions that he wasn't under investigation (which, in itself, would be inappropriate since Comey shouldn't be telling the President anything about such an ongoing investigation).