I wasn't there. Guess 1: perhaps you saw a failure to mount a valid root filesystem. Guess 2: perhaps you jumped releases and were missing the new release's signify(1) public key.
The install-or-upgrade script can be found in the source tree, in two parts. The main functions are in /usr/src/distrib/miniroot/install.sh and the architecture dependent components are in /usr/src/distrib/<arch>/common/install.md.