What happened here?

This past weekend we were getting ready to head to Va Beach to see our son and I went down to the basement to get some beef (a neighbor and I raised two Black Angus bulls) to take to him.  When I went to turn the light off I looked out the window and saw this...

Now how did that happen?  I checked the fencing and all looked good, so I recused her and off we went.  Later in the day my daughter called and says the other dog was in the coop and all the chickens were gone except for the Rooster.  She let the dog out and found the place in the fence that was broken.  A quick repair was made till I got home the following day.  We were missing two chicken, the rest had been saved and put back.  Over the next few days only one we found alive and well in a bush.  Still one unaccounted for.
UPDATE:  Last chicken found in neighbors bushes, spotted by their dog.  She is home safe and sound.