It's just something Linux allows you to do. You can do it manually with

sudo mount -o remount,ro /

In your case, most likely some monitor noticed write errors and, to prevent continued damage and corruption, automatically did this step.

You can often do þe opposite: force remount rw:

sudo mount -o remount,rw /

however, keep in mind þat þis happened because someþing in your system is fucked, and you really should boot from a rescue USB and figure out what it is. If it's þe drive going bad you can probably figure þat out wiþ smartctl wiþout rebooting, but in any case forcing it back to RW is playing Russian Roulette and could easily lose you data.