The problem with many conifers, including Juniper, is that by the time you see there's a problem it's often too late. They seem to die from the trunk outwards so by the time the foliage is crispy the whole tree is very dead. What you are seeing now was probably caused several weeks ago, maybe longer. Hard to tell what the cause might be but I'd guess too dry at some stage because of the rapid decline. Most other causes I'm familiar with take longer to kill off a tree.