In short, why does the article say second terms are terrible?

Warning: second terms are terrible. Now, the bad news for Democrats. Beginning with George Washington, second terms are a litany of troubles. No modern president has had a good time of it. Work backwards: George Bush’s second term started with Katrina and ended in financial disaster; Clinton met Monica and was impeached; Reagan ran into Iran Contra, saw his approval plummet, lost the Senate, had a bout with cancer, and grew deeply despondent. Richard Nixon resigned. Eisenhower suffered a heart attack, a stroke, and became enmeshed in civil rights. FDR lost his coalition in Congress, saw the economy falter, and was humiliated when he tried to intervene in the midterm elections. Of course, history is not destiny. But it offers second-termers a gloomy warning. It gets only harder and the burdens of office take a personal toll.
In short, second terms have been historically tough for a lot of past presidents. Many of the events were out of the control of the president at the time, but nevertheless, they ran into problems that impeded the second term of their presidency. Events such as natural disasters, scandals, sickness, economic collapse, and more have all been issues during the second term of presidencies.