After further investigation into the matter here is a breakdown of the causes for the 12 special elections held during the 101st Congess: 5 members left the House for new positions, 3 resigned amid scandals, 2 died of natural causes, and in some sort of strange unfortunate coincidence 2 congressmen died in separate plane crashes in the same week of August 1989.
Mickey Leland of Texas and Larkin Smith of Mississippi both died in plane crashes.
Of the three districts that switched parties, two went from Rep to Dem(MS-05, IN-04) and one went from Dem to Rep(FL-18).