What is the cause of the large decrease in electric utility emissions, particularly sulfur dioxide, starting in 1995?

SO2 emissions reductions from utilities during this time are primarily attributed to the impacts of the acid rain program.  As described on EPA’s Acid Rain Program web page, Phase 1 of this program began in 1995 and affected 263 units at 110 most coal-burning electric utility plants in 21 eastern and midwestern states.  As you point out, the acid rain program was implemented via Title IV of the 1990 Amendments to the Clean Air Act.