Something tells me that Beoing didn't design the F/A-18 gear doors to be held open when the plane does a dirty roll with gear extended on take-off. Looking at the door and looking at the direction of the roll, I can understand that gravity and centrifugal forces would slam that gear door into the landing gear and eventually weaken the metal and cause it to break off.
Since all of the Blue Angel planes do several manuvers with the gear extended and plane #5 does a lot of them while inverted, it is very understandable that these doors would break like that. I guess the Blue Angels are lucky that when the metal does break off, it doesn't go into the gear well or hit other areas and cause more damage or even a dangerous failure.
They may not have built the aircraft to perform these maneuvers, but they WERE designed to make hundreds if not thousands of carrier landings. As Liza said these Hornets are the same age as some of the first Hornets that the Navy flew. Banged up sheet metal is nothing new to these guys.
Also, if you've ever seen a video of the F/A-18s gear coming down, the rear wheels will actually jolt or kick at a point on the way down. A slow door that got in the way would be a precursor to that door getting dislodged.