Running a fill destroy command makes the break particles on all the blocks appear at the same coordinate at the upper south-east corner of the fill command.

To test:

1. Place some blocks in the region you are filling
2. /fill ~ ~ ~ ~5 ~5 ~5 air 0 destroy (or use whatever coordinates you wish)
3. Notice that the particles all appear in the upper south-east corner no matter where you fill from.
4. /fill 0 0 0 5 5 5 air 0 destroy (or use whatever coordinates you wish)
5. Notice the particles again appear in the upper south-east corner