If they are using DC power supplies this is a known issue on PR 1064039 where the fans do not detect the temperature correctly and fail to cool the power supply causing the shutdown to occur.
This is fixed in Junos 13.3R7
If you are not in a position to upgrade, JTAC can give you a SLAX script to periodically test the temperature and run the fans. You may need to adjust the temperature thresholds in this script for your environment to prevent the power supply failures. We found the settings too high initially but the script does work for systems not yet upgraded.