Answer: Based on your description I am not sure what happened to your trees. Trees can seem to die suddenly when the real cause is that a long term stress situation has finally caught up with them.
I don't think the spikes would have killed them suddenly unless somehow they were terribly overfertilized. So I would look at other factors such as accidental herbicide contact, drought stress the previous year or two, vole damage to the roots, rabbit damage to the bark, poor rooting, soil compaction due to contruction, excessively wet or dry soil, things like that, or possibly a pest or disease problem.
Rusty leaves and dieback sound more like a disease issue such as cedar apple rust or if the dieback is accompanied by a blackened, burnt look then possibly fireblight. I would suggest you carefully clean up and remove all the debris from the dead trees to try to limit sources of reinfection. Then, keep an eye on the tree that is left. If it begins to show symptoms, contact your local county extension to obtain a specific diagnosis of the problem and based on knowing that, determine how to proceed.
You may also want to check in with them for their recommendations for preventive spray programs. What to use and when to use it depends on the cultivar of apple you are growing, the prevalence of diseases in your area, and the weather each year. Some sprays are actually applied in winter, so you need to have a plan and schedule. Finally, they should also be able to recommend the best disease resistant varieties for planting in your local area. I'm sorry I can't be very specific for you but I hope this helps you start troubleshooting.
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