UNKNOWN
Well I won't be able to give the reasons why the second volley took place.  As I stated in my statement that my only assumption is the fact that this was another ploy to wreak maximum havoc, to actually take people by surprise when all of us think that it is all over and we begin to stand up and walk.
 For instance, myself, if I were to explain my personal experience, I was actually shouted down by a comrade that was next to me because I was standing up to go and assist a comrade that was gunned down in front of me and I was told that I must lie down again.  And at that very time the second volley had actually started.  So to me in my own assumption, this was another ploy to wreak maximum havoc.