Filesystem became read only, can you tell us why ?

Issue

• We tried to upgrade firmware first, the first two days are good with no random reboot. On the third day a number of filesystem (include /var and /) suddenly become read only, and a reboot fixed it.
• We don't see any special trace on the host why the filesystem become readonly. Would you be able to suggest why or suggest how to find out more on the host regarding this issue?