It could be anything. A good start is to look at... the log.

What was in the log?

if this happens again it's probably a good idea to check the logs.

also try logrotate

be mindful though tgat logging if not configured right can put a lot of wear on your SSD, so it might be an idea to set logrotate to Something rather small and mount your log directory (if you generally don't need logs to survive a reboot) to a tmpfs Mount(if you've got some RAM to spare)

You probably have debug logging enabled somewhere.