You can always ask why; but the real question I think is whether management is required to respond to your query in order to require your cooperation. And unless you are covered under a union contract that specifies differently, the answer to that question is: No, it is not.

That is, management has a right to demand your cooperation regarding a meeting at any time for any reason during hours for which you are being paid your normal rate. Management is not required to provide you with a reason for taking away your hours or for your attendance at a meeting.

Good management practices might indicate that it SHOULD tell you why your hours were given to someone else and the reason you are being told to report to a private meeting. But your question pertained to rights, and management is not required to do either of these things.