This is done automatically when the drive has a failure that the OS can not recover from. It remounts it to prevent catastrophic damage to the filesystem so that you have a chance to copy off any data that is still salvageable.
If you have S.M.A.R.T tools (e.g. gsmartcontrol) loaded in dom0 you can inspect the drive to determine what caused the error if the drive supports it. There will be logs and statistics which will explain what is going on with that drive.
I have had many failures like this when my backups go offline because the drive switches to r/o. Certain brands of drives are notorious for certain kinds of errors such as spin-up/start failure being returned to the controller after the disk goes to sleep from drive inactivity. The problem usually just gets worse the more you use it.
While you can probably just reboot to get a questionable drive back to r/w it’s not advisable to continue using that drive for backups for any important machine worth backing up.