BD-ROM Drive suddenly appeared in My Computer

The likely explanation would be that you installed virtual drive emulation software such as Daemon Tools, Alcohol 52%/120%, or PowerISO (or another disc-burning software that has drive-emulation capability).

In most cases depending on the version and program used, a virtual drive is added automatically after install.

BD-ROM DRIVE means Blu-Ray Disk Read Only Memory Drive.

It usually appears when you install a Virtual Drive using softwares like Daemon Tools, PowerISO, etc, and it is used to mount .ISO files and many other data formats.

In Daemon Tools you can also click on "Remove all drives" and this will get rid of all the unwanted BD-ROM drives