Bad master directory block error (shutdown cause -60) and random restarts without other symptoms
@Allan suggested that the symptoms I reported were consistent with a failing SATA cable, which is a known issue. He suggested that I check the disk status with some utilities. I could find no disk issues when I did this.

The post I linked to earlier suggested resetting the SMC in addition to checking for disk errors. I reset the SMC. I have not experienced any unexplained shutdowns since. Any further errors, and I'll investigate that SATA cable.