Mugo pines are an excellent choice for a sunny hillside! They appreciate full sun and a hillside planting generally allows for good drainage. It is possible that your watering may be off - either too much or too little. How often and for how long do you run the soaker?

Right now the soaker is running for 30 minutes each morning at 7am. The dead areas suddenly showed up out of nowhere with only a recent increase in temperatures. Do you think there's a way to reverse this?

Daily irrigation for a short period of time - especially with soakers - is seldom a very good idea. In fact, most sources that discuss proper irrigation for plants in the landscape will suggest a less frequent but much longer application of water. It would appear that these plants are not receiving sufficient water. The problem is that soakers (and typically drip systems as well) emit water at such a low volume that an extended period of time is required for sufficient water to be produced and migrate down through the soil profile to the proper depth. With each watering, you want the water to percolate down to at least 8" but deeper is even better. More frequent but short watering periods only produces moisture at the soil surface, not down throughout the root zone where the plants can properly absorb it.

With soakers in my garden - and I am NOT in a hot climate - I leave them on 4-6 hours or overnight. But I only do that once very 7-10 days, often a bit longer. You will have to do some testing to see how long you need to leave the soakers on based on tour soil and climate conditions but I'd suggest no more than once a week and for a period long enough for the water to penetrate down at least 8-10 inches with each single watering!!

Btw, once these plants are properly established in the landscape - 2-3 seasons of growth in place - they should only require very infrequent watering and only during very dry periods. Mugos are very drought tolerant once established.