24" box size trees need more than 2 gallons of water a week to establish themselves in the ground, even desert trees. Did you get any instructions on watering from whoever planted it?

As desert plants, palo verdes will be dormant in any season if they have no soil moisture. Your tree maybe be going dormant because it's not getting enough water or because it is young, newly planted, and stressed. We had a long stretch of cool weather, but now the summer heat has hit hard and it is responding by dropping its flowers, to protect itself and conserve moisture. A mulch would keep the soil cooler, hold moisture in, and reduce stress on the tree. Keep mulch away from direct contact with the base of the tree. I'm assuming you have 'Desert Museum' which is what is being widely sold in CA.

'Desert Museum' care

Drought tolerant trees and shrubs aren't drought tolerant the minute you put them in the ground. They take at least a year to establish a big enough root system to gradually wean them off of regular water. That's why fall is the best time to plant, especially something as large as a 24" box. Follow the directions in the post above. I wonder if you bought the tree from a place named after a celestial object?

They need 2 years, minimum, and that's for a 15-gallon.

And the bigger they are at planting time, the longer they need supplemental irrigation.