That very likely is just transplant shock. Transplanting large trees like that is not easy. Roots are dug up and broken in the process. And sometimes they make it through that and some times they don't. I hope you got these nice and obviously expensive specimens from some place that gives some guarantee. Did the nursery you bought them from install (plant) them or did you? If you did it, please tell us some more about how you planted them (how big a hole, etc).
You are right that you need to keep the plant from ever drying out. But you also have to avoid letting it be soggy. Over-watering can cause root rot. It should get about an inch of water a week, including rainfall*. Overwatering can cause root rot that also can cause foliage to brown and die back, so water only when the top few inches of soil feel dry to the touch.
It looks like you have some mulch down, but it looks like only a thin layer. I think you need a lot more. You want a 3 inch layer or so of mulch. That will help conserve the soil moisture and keep it more consistent, rather than going through so much wet/ dry cycles, which are hard on the tree.
Keep an eye on the browning to see whether it is getting worse and watch for new bright green growth on the tips. If they are putting out new growth, that is a sign that they are rooting in and probably will make it.
If these for some reason don't make it, my suggestion next time would be get smaller/ younger specimens. Younger ones transplant much easier and take off quicker. Often in two years, the smaller ones have caught up to ones that were transplanted at a much larger size, but struggled more.
Best wishes! Keep us posted on how they do.
* If you didn't get any rain, one inch of rainfall equals 4.7 gallons of water per square yard. So that means you need to put at least five gallons of water on the soil around each tree, every week that you don't have any rain.