Simple chemistry is the answer.
Don't get me wrong, it would have been far better for the first group to have had duds as well.
P.S. But remember ...that is why i searched for acetone peroxide. has probably been recorded by some NSA computer and your Internet traffic will likely be monitored for quite a while. So, behave yourself!
what i don't get is why the first group were suicide bombers and the second group apparently was not. so i am cautious about thinking that both sets of bombs were the same....