The most common cause would be a tripped boot counter. If an SP reboots for the same reason a certain number of times, it will go into Service Mode.  You can check the status over CLI with the service command: svc_rescue_state -l  In this list you will see the "count" columns which tells you the current value for your system. The "max" and "action" columns tell you what will happen (action) if the max is reached. Example: 3 appinit type reboots will put the SP in service mode (or rescue mode).  The first thing to try in these cases is to clear the counters and reboot the SP: svc_rescue_state -c svc_shutdown -r  If the SP continues rebooting after this (rolling reboots) then it's time to open an SR.
Hi all,  What is cause to reach service mode?
Unity service mode