PROBABLY not enough water. But I would dig a hole down at least 6" next to the bush and see if the soil is dry or moist. It may be that a mole, vole or gopher tunneled through the roots and, for the last 2, ate as it went. Either way, roots don't do well in plain air - so that might be part of the trouble.

I would also do a scratch test - gently use a fingernail to scratch the top layer of bark off a small branch. If it is brown all the way to the wood, then that branch is dead at that point. I would test in several places, closer to the trunk, if that is the case. If it is still green under there (the cambium layer) then I would see about the soil, but would bet that it is too dry. Start soaking it, and give the others a bit of extra water as well - long slow soak for them all, but repeat for the problem one at more frequent intervals. Indian Hawthorns are tough, but can suffer from too little water. If that is all (ALL!!!) the problem is, then it should come back with new leaves when given plenty of water.

It may be that that area of your yard has slightly different soil than the other areas, and so it has a problem with moisture there - maybe a dumping of builder's sand, rubble, etc., which has been buried since the house was built. If that is so, then you will need to pay extra attention to anything planted there during drought-y conditions.

Thank you, dibbit. I really appreciate your comment and advice. We have just recently discovered what we believe are voles which have dug at least one hole nearby. Along with the slight drought and heat, our home was constructed 2 years ago, and so the comment about buried construction material is pertinent too. We'll try your advice tomorrow morning. Again, thanks.