As you can see in the accompanying photo, there is quite a large bur on the exit side of the previously drilled hole. Not all burs are this noticeable, but they are always there after machining operations. You may not even notice most burs by sight, only by touch. This is why it is so important to clean your material after each machining operation.
In the accompanying photo, notice the efficient function of a riffling file; there is just a small concentrated area of the file in contact with the surface of the material.
Imagine all the burs that were created as the drill bit passed through from one material layer into the next. The burs can and will actually cause separation of material, even under the pressure of the clamps. The material has to go somewhere! So from here on out, get used to this mantra: Disassemble, de-bur and reassemble!