According to the knowledge base, a volume can become read-only after the storage pool has previously had an error, and Synology recommends the following:
1. On the active server, go to Storage Manager to find which storage pool is read-only.
2. Go to Storage Manager > HDD/SSD to find which drive is read-only and replace it with a healthy one.
3. Go to Storage Manager > Storage. Select the storage pool where the read-only volume belongs and click Repair.
After the volume is repaired, the system needs to synchronize all data from the active server to the passive server. During synchronization, switchover and auto-failover are not available.
If your issue is specifically a volume becoming read-only in DSM 7.0 and above, the knowledge base also says that for hardware malfunctions you should back up your data and remove the read-only volume to recreate a volume.
Sources used: [1], [7]