Student or Learner
What does this sentence mean? Why 'out of control' stands after 'rage'?
"Rage" and "out of control" are so often used with one another that together they stand as a cliche. "The wildfire raged out of control across the state this weekend."
Here, they are being used humorously to exaggerate the condition of the "in-tray." I'm assuming you already know that an "in-tray" is literally a container, most often shallow and rectangular, used to receive "incoming" work; a secretary, for example, might have an in-tray on his desk into which his boss places letters to be answered, bills to be paid, etc. Depending on what you're reading (and I'm betting it's a self-help article or book) the "in-tray" can almost stand as a metaphor for the list of tasks that other people are putting on you.
Anyway, an in-tray raging out of control is simply one that is ungovernable, one filled with a mountain of paper that you can never hope to conquer. Surely you know the sensation :)