Specifically in San Diego county, the state-wide ban on hunting the mountain lion caused the population to expand beyond the relatively small carrying capacity of the habitat in the mountains, and then they ate far too many of the deer. Then the young cats couldn't find a hunting territory in the mountains. The area to the west was too developed, so they were forced to the east into the Anza-Borrego desert, where they proceeded to kill and eat the desert bighorn sheep.
The mountain lions were not endangered.
The deer, which formed the lions' usual prey in the mountains, were not endangered.
The desert bighorn sheep were definitely an endangered species, with a very small population in the San Diego and Imperial counties. They were all fitted with full biometrics, so there's no doubt how they died; the lions ate them.
Needless to say, you won't read about this in Wikipedia, or hear it from the Sierra Club.
Some other important things to understand -
Hunting & Fishing licenses are the primary source of revenue for wildlife management.
The State Fish and Game department had the situation in San Diego county well in control before the ban. They monitored the population of the deer and the lions, and issued an appropriate number of hunting tags to keep the populations stable and in the correct proportions. The lion tags were hugely expensive, and the success rate was only about 2%, so they generated a lot of revenue for wildlife management projects.
Now there's no lion hunt, so no sales of lion tags, so no revenue. The deer herd has been devastated by the booming population of cats, so they can't really sell many deer tags at all. Hence the department of F&G has less money for wildlife management projects in the south.
See what I mean when I say it's complicated? There are so many factors to be kept all in delicate balance. But we had a bunch of emo amateurs crying about "horrible hunters slaughtering those poor kitties", and they got their cat hunting ban, but the deer and the sheep and all the other wildlife management projects got screwed. Because people like that never think beyond the obvious surface layer of the issue.