They would have been lying all around on the ground. It would have been so easy to pick up a stone, maybe not a large one and throw it at someone you disagreed with. Then when it was seen that you had hit the mark someone else would pick up a larger one and hurl that. Then the whole action would gather momentum. More and more stones would have been thrown.
It is unlikely that the original idea was to kill Stephen but crowds easily get out of hand and common sense goes out of the window. It could have been in this way that Stephen became the first martyr. He had to be stopped from continuing what he was saying. So died Stephen, the first martyr but definitely not the last.