That mode indicates that the boot ROM failed to verify the signature on the bootloader. Generally this means that for some reason your bootloader got corrupted. The way to restore this is a special qualcomm tool, but you also need a properly formatted and signed package to flash using the tool, and i doubt that google would give it to you.
How did it get corrupted? FTL issue in an aging eMMC? Bug in linux kernel block device driver? Malicious code? Sheer dumb misfortune? Who knows...
How did it get corrupted? FTL issue in an aging eMMC? Bug in linux kernel block device driver? Malicious code? Sheer dumb misfortune? Who knows...