What connects the predecessors of Andrew Johnson and Lyndon Johnson as US Presidents?

Both predecessors were assassinated.

Abraham Lincoln - President Lincoln was shot April 14, 1865 at Ford's Theatre in Washington, D.C. while watching the play Our American Cousin. He died the following morning at a boarding house across from the theater. Andrew Johnson (December 29, 1808 – July 31, 1875) was the 17th President of the United States, serving from 1865 to 1869. Johnson became president as he was vice president at the time of the assassination of Abraham Lincoln.

John F. Kennedy - President Kennedy was shot and died on November 22, 1963 in Dallas Texas. Although there is much controversy about the assassination the Warren Commission concluded the assassination was the work of a lone assassin; Lee Harvey Oswald. Lyndon Baines Johnson (August 27, 1908 – January 22, 1973), often referred to as LBJ, was an American politician who served as the 36th President of the United States from 1963 to 1969, assuming the office after serving as the 37th Vice President of the United States from 1961 to 1963.