The petals seem to not be falling off exactly (unless you're seeing them scattered on the ground in large numbers) but rather chewed off. This is a very common phenomenon. Various insects can be responsible, though none of them are serious because the flowers still have value to pollinators (and later, if not removed, as seeds for birds in late summer, fall, and winter). Candidate chewers include caterpillars, beetles, earwigs, young grasshoppers, crickets, and young katydids, several of which blend-in well or feed at night, making them hard to catch in the act.

No insecticide treatment is needed, especially since using one could potentially harm the pollinators the flowers are meant to attract. The plants will continue to bloom as their energy stores allow, and all that needs to be done to support them is to monitor their watering needs as our current drought continues. (Feel the soil around five inches deep and water well once it becomes somewhat dry to the touch at that depth.)

Miri

Miri,

You're welcome.