The green shrubs either side of the dead-looking ones look to have some brown leaves adjacent to the dead ones, unless they are intermingled. Are you saying that the insects that are in the first picture are not on the dead shrubs at all? None of the pictures are clear enough or close enough to identify the problem or the insects, and without knowing what kind of shrubs they are we can't look up what they are prone to.
To answer your question about spreading, I have seen individual plants badly affected by scale insects next to ones that they haven't yet moved onto. But if there is no sign of the insects on the dead plants then that doesn't seem like the problem.