Suddenly YHWH said to Moses and to Aaron and Miriam, “Come out, you three, to the tent of meeting.” And the three of them came out.
Numbers 12:4

"Suddenly" should be understood as "sharply". God addressed all three of them harshly, indicating that Aaron and Miriam were definitely in the wrong, but Moses must have contributed to the problem in some way. Maybe Moses was a little *too* meek in his leadership role.