Have you reviewed the CAM simulation for potential collisions? Which bur was it specifically?
My guess with the given information is that you broke either the .6 or 1mm bur and that it broke when encountering material that wasn't fully cleared by a previous bur. Of the dental materials commonly milled PMMA is one of the most likely to break burs, so a collision that may have been ok in wax or zirconia may break a bur in PMMA and the age of the bur might not make a difference.
So I would start with review of the CAM simulation. If you are not familiar with the CAM simulation, or what you are looking for in it, I would recommend calling whoever supports the CAM software that comes with your mill. The solution may lie in using a different strategy or changing the orientation and sprue placement of your nesting.