Personification occurs when authors give inanimate things human characteristics.
At the beginning of the novel the boys are one complete whole. They are as the rocks are where the "rock makes a sort of bridge" (112). It is at this point in the novel where the group of boys start to split into two factions of boys, (Jack's hunters and Ralph's crew) just as "the rock of the cliff was split and the top was littered with great clumps that seemed to totter" (113). Besides Ralph, Jack, and Roger, the boys are like the rocks (or the rocks are like the boys-easily toppled) with each of the two boy's factions "rocking the rocks" until they fall down on one side or another--or the boys fall down on one faction or another.It is as if the boys are choosing reason or emotion, good or evil, morality or immorality. A survey of LOTF's themes would help us here.
On another level, the boys would much rather roll rocks and cause destruction than look to securing their rescue from the island because they ignore the importance of re-lighting the signal fire.