Detail I tracked down when this was posted yesterday, reposted here for a bit of additional info:
Prestige Biotech was a creditor for and acquired the assets of Universal Meditech when the company collapsed. Prestige was storing what was left of Universal in this warehouse.
Universal Meditech was selling SARS test kits that got recalled in February of last year, which likely led to their bankruptcy. My guess is that Prestige didn't have the available funding to properly dispose of the biohazards or care for the test subjects, ended up with a bunch of infectious mice that they couldn't get rid of legally, and just decided to warehouse them in Fresno until they came up with a solution.
The OP's article adds the important detail that Prestige wasn't licensed in California, so I theorize this may become a criminal case, especially given the press it's getting.