If the kernel finds fatal corruption on the disk or if certain key IOs like journal writes start failing, the kernel may remount the filesystem as read-only. This is because the filesystem can no longer maintain write integrity under these conditions. Any such behavior will be thoroughly logged in /var/log/messages.
Should this happen, backup your recent data as this may be a symptom of an impending disk failure. Perform filesystem checks on the disk using e2fsck as soon as possible and use the -c option to enable badblock checking. The normal fsck may not detect all the errors and return clean. For example:
e2fsck -c /dev/sda3