Y'all have had a wetter than average year in So Cal. That kind of weather tends to bring out ticks. The baby ones are very hard to spot, so you might never even notice if the dog picks one up.
Parasite issues can change rapidly in the environment. Here's an example, from several years back in So Cal: When I lived in So Cal, almost no one kept dogs on HW prevention.The vets offered it, admitted they never saw cases of HW, and that was usually that. Then the year after Hurricane Katrina, multiple dogs in my neighborhood came up with HW disease. There had been flood dogs from New Orleans fostered locally, and they likely transmitted it to the mosquitoes who started spreading it around. Suddenly, every dog really needed to be on HW prevention!
Along the same lines, hardly anyone in So Cal uses tick prevention because it's not usually an issue. Until the year when it becomes an issue....and this might be the year.
Where I live now is an area where vets tend to say "we don't have tick problems." I have never seen a tick on my dogs. Many vets in NOLA and Baton Rouge don't bother recommending tick prevention -- most people simply don't need it, they say. However...our rescue has treated MANY cases of confirmed tick disease in the last two years from dogs coming out of Baton Rouge and New Orleans. So the ticks have to be around -- and they're spreading disease....even though they're not "supposed" to be in the city.
Our rescue's vet thinks a lot of local tick disease cases in areas like ours aren't being diagnosed properly because the symptoms are so vague and the local vets are assuming something that used to be true about the environmental risk is still true -- and it's not, as demonstrated by all the tick profile results we're getting in bloodwork we send out.