According to wiki.

The word bulki meant “cargo” in Old Norse. Sometime in the 15th century sailors and builders realized that walls within a vessel would prevent cargo from shifting during passage. In shipbuilding, any vertical panel was called a “head.” So walls installed abeam (side-to-side) in a vessel’s hull were called “bulkheads.” Now, by extension, the term applies to every vertical panel aboard a ship, except for the hull itself.

The term was later applied to other vehicles, such as trams, aircraft or spacecraft, as well as to containers, such as fuel tanks. In some of these cases bulkheads are airtight to prevent air leakage or the spread of a fire.

Other kinds of partition elements within a ship are decks and deckheads.

That still doesn’t answer the question of why they call it that. Maybe there’s a damn interesting story behind it?