This happens to me frequently, that one or two hives out of several right next to each other will experience pesticide die-off and the others are not affected. I opened two pollen traps this week right next to each other and the color mixtures were radically different. One had lots of purple pollen which I assume comes from a large stand of thistle which I've seen bees working, and it was really pretty. It just shows you how different hives, especially when there is a large choice of nectar, can visit different areas. Perhaps someone didn't like your bees on her escallonias or his poppies and sprayed.