It's hard to diagnose the exact cause of branch dieback, but environmental stress is a likely cause. Perhaps drought stress (depending on how well the irrigation system hydrates the roots), over-watering or compacted soil, or other features of the site are making the shrubs vulnerable. While several pests and diseases can afflict azaleas and rhododendrons (see the link to our diagnostic page), we don't see clear indications any insect or fungus is at work here. One exception might be Botryosphaeria canker, though the only recourse for that once it appears is to do what it sounds like you have already been doing -- pruning out the dead branches. If present, opportunistic pathogens like that canker fungus take advantage of plants previously stressed by drought, high heat, or other environmental conditions.

If you wanted to see if the irrigation system is helping, feel the soil around five inches deep in the root zone in order to monitor the shrubs for watering needs. If somewhat dry to the touch at that depth, a thorough watering is probably needed to re-moisten the soil back to that depth. If damp when checked, watering is probably not needed yet. If the system runs on a timer or set schedule, that can easily over- or under-water plants, since it's not responding to soil moisture levels or plant needs in that case.

Miri