As per Dictionary.com, “burg” is commonly used in reference to a city.

It is usually used at the end of a word that will reference the founders or original reasons for the existence of the city.

In the case of JohannesBURG, it is unclear and debatable what led to its establishment. What historians do agree on is that behind the name is one Johann Rissik, who served in the office of the surveyor-general. This was during the time that gold deposits were discovered and the rush for not only that gold but also the piece of dirt on which would be founded a world-class city. Johann Rissik and Christiaan Johannes Joubert were sent by the state to check out the situation and start planning for a formal township to be established.
Google “berg” and you’ll come up with something like:

Now in JohannesBURG lies an International Airport that serves as a major hub into and out of Africa. A kind of big deal. JohannesBURG is a name that’s commonly used in all social circles and from early school years. Whether you prefer Jo’burg or Johannesburg, berg doesn’t come into it. We must speak proper.