They accepted more tests than they had the ability to quickly process. They also appear to have accepted many tests that required use of a lower throughput assay before switching to higher throughput testing on Roche 8800s: https://www.theatlantic.com/health/archive/2020/03/next-covi...
Thanks, googling found this[1] statement by Quest that they have a backlog of 115,000 tests, down from 160,000 on March 25th. It would be interesting to know what percentage of the California backlog is contained within that.