Why are they almost always bad.
Bush- Katrina, Iraq
Clinton- Impeached
Reagan- Iran contra
Nixon- Watergate
LBJ*- Vietnam
Ike- Economic downturn. GOP loses very badly in '58
Truman*- Korea
FDR- Supreme Court packing plan
Wilson- League of Nations failure.
Coincidence or something more.
*Not a second term. But were incumbents who were reelected.
You're forgetting that a lot of these Presidents also had problems in their first terms. FDR had the Great Depression, Truman had a severe recession and hyperinflation, Ike had a recession, Nixon had a recession and high inflation, Reagan had a severe recession, Clinton had a miserable screwup on healthcare reform, and Bush Jr. had a recession and 9/11. Also, a lot of the problems Presidents experienced in their second terms began in their first terms--such as Vietnam, Watergate, Iran-Contra, Clinton's affair, and the Iraq War. However, it does seem that in many cases the Presidents appear to care less and take more risks in their second terms simply due to the facts that they have nothing to lose electorally anymore.
Excuses, excuses. The fact of the matter is that all of the presidents listed sucked.