maybe I found the cause myself:|
The boot messages found in syslog after the cron.daily have a timestamp shortly after the cron.daily entry (which saves the current time to the fake-hwclock file).
So this is not because the reboot happened shortly after cron.daily, but because the pi hung until I did the manual reboot.
Because there is no real hwclock, the boot messages have the timestamps of the last fake-hwclock filewrite.
It does not boot randomly , it hangs randomly.
One problem solved, another (worse) problem found