Redo log fill up the mount point

We are using db version 11.2.0.4

One day, our redo log suddenly fill up the mount point and crash the database. I thought this is pretty odd. We have Plenty of OS disk space in that mount point to begin with. Redo log does not autoextend. Even through if we have very large transaction and millions of row update. I would think it will just do frequent check points and generate lots of archive log. So, if archive log fill up the mount point, I can understand. But why redo log? I am just puzzled. Could gurus explain to me in what situation can redo log