In most situations, the only way to know if the handwriting is on the wall is to speak with the hand-writer. Your boss's actions could be part of a larger restructuring, and perhaps he believes that your current responsibilities fill your plate, and he would like you to focus more fully on them. Or, perhaps this is the first step in dismantling your position and ultimately your job.
Your boss is not showing much managerial finesse. One could easily argue that he should have discussed this type of change with you prior to making his decision. The fact that he has yet to mention the change to you raises further questions about his skills and motives.
Nonetheless, although he is not particularly communicative, that does not mean that you should be uncommunicative, too. You should meet with him with the objective of clarifying your current responsibilities, the rationale behind transferring some responsibilities to another manager, and where you and your position are going. By taking these kinds of actions, you will know if the handwriting on the wall is actually an exit sign.
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