A tale of misplaced insulation, leaky ductopi, and lazy builders
The problems you are seeing here are due to a combination of three problems:
- Vented attics in hot climates get scorchingly hot
- Your ducts are in the attic (and are a lazily-installed "ductopus" of flex, at that)
- Your ducts, sadly, leak (somewhere)
Fixing the leak will stop the hot air from coming into the house, but it won't cure anything else that's wrong with this setup, as the unstretched flex hampers airflow through the ducts and the R8 or so duct insulation is nowhere near enough to keep attic heat from warming the cold air in the ductwork, either. Considering that there's no ice dams in SoCal, hence no reason to vent the roof deck otherwise, I would go to an unvented conditioned attic, or "hot roof", configuration.
You can either have a couple thick layers of rigid insulation board laid down with the gaps staggered and an overdeck laid over that next time you replace your shingles, spray foam applied to the underside of the existing roof deck if you want to do the retrofit now, or a mix of the two, provided you achieve the R-value you're after. If you are doing spray foam on the underside of the existing roof deck, you'll also want to put some 1/2" drywall on the underside of the rafters to provide a thermal barrier between the foam and any sort of flare-up that might happen in the attic (some folks use a sort of paint-on coating instead, but the drywall is easier to get right and maintain I reckon). Of course, the attic venting will have to be removed and replaced with solid materials instead, and supply and return grilles added to the attic so that the air conditioner can cool it.
While you're at it, you'll want to have that "ductopus" of flex replaced with a rigid trunk-and-branch duct system, reducing the amount of work your air handler has to do to push conditioned air through the system. Between these two upgrades, you'll be able to take a good 10-25% off of the energy usage of your air conditioner, and may even be able to downsize the A/C next time you are shopping for one, since your house no longer wastes effort trying to cool outside air that's just going to get vented anyway, and your air handler no longer has to push as hard to get cold air to you, either.
(As a sidebar: it is possible to "bury" the ducts in attic insulation, but that's trickier than cathedralizing the attic due to condensation issues; it requires careful spray foaming of the ducts in order to keep the dewpoint on the outside of the vapor-impermeable section of the system high enough that humidity in the outside air doesn't go off and condense on the outside of the ducts, creating a moisture problem.)