The Roman term for a dictator was an actual political office and was somewhat a common occurrence. Sulla was certainly not the first! During the early Republic, dictators were often appointed. Now, this is a wikipedia chart, which you know, isn't a great source! But it will serve its purpose here.

http://en.wikipedia.org/wiki/List_of_Roman_dictators

That is a list of Roman dictators. For the reason they were appointed you will see many instances of Rei gerundae causa. This translates to for the matter to be done. These men were often appointed for wartime duties. Many of them, almost all of them, resigned willingly. A particularly famous dictator was Cincinnatus, who was made Dictator twice in his lifetime for the position of dictator, once to war waith two other tribes on the peninsula the Aequi and Sabine and the second time for another one of the reasons you see listed, Seditionis sedandae et rei gerendae causa, though that article lists it wrong, it isn't an or clause, it is all together. This means the putting down of rebellion and for the matter at hand. He was appointed this second time to deal with a man who was attempting to become king. Every time at the finish of his duties he retired back to his farm! Cincinnatus was a model for civic virtue and I like to call him the Roman Washington, or perhaps it should be the reverse! Livy has quite the love affair with him (like most Romans) and writes about him in some of his historical works.

Now, after the Civil War period later emperors shied away from the using the term because of Sulla and Caesar. As you can see there were also other reasons dictators would be appointed, often for religious or civil things. Now as for why people would say that about Sulla, look at the dates! Sulla was the dictator during the Social war. I won't get into all that as its a whole other thing but Sulla sort of seized the dictatorship which made his a bit different, it also had been a long time since the last dictator.