Large pieces of plant quickly dying seems like a root fungus - water mold situation, to me. When it was 103, was the ground soggy? That might have started things. When big chunks die back, I try to propagate the remainder...

Moisture plus heat is also conducive to bacteria, but I don't know about greggii's susceptibilities that way...

Jon

I've had that happen. I thought it was either the plant trying to protect itself by giving up a branch, OR, that branch was one of the older ones and was just it's time.

Jonapp , the ground was quite moist ay the time of the die back.We do have lots of strange oak fungus appear from the ground from time to time,especially summer. At the moment the rest of the plant is still alive . I gave it a light haircut to take off all the blooms,thinking it might give more strength to the remaining plant

We have the same problem here. It is started by Spittle Bugs (one of many vector insects) showing up. By sucking juices out of plants(which looks like spit) they ofter transmit diseases from plant to plant. Very common to Salvia, Rosemary and Juniper.If no insect is present you may have a fungus called Collar Rot but this usually kills the whole plant not just branches.
Art