Protesters turned a seemingly routine vote to begin calculating PG&E Corp.’s wildfire costs into a rally against the embattled utility Thursday at a California Public Utilities Commission meeting.

Drowned out by cries of “murder,” “perjury” and “manslaughter” from about a dozen protesters, commissioners led by CPUC President Michael Picker unanimously agreed to open proceedings to develop a so-called stress test for utilities’ wildfire costs. The goal is to see how big a financial blow companies like PG&E could withstand and...