Lack of motivation. Wilander realised that he would have to work even harder if he was going to stay as world number 1, and he didn't have the desire to do that. He said in 1989 that he got more enjoyment out of cutting the lawn at his house and playing the guitar, than in playing tennis.
After Wilander won the 1988 US Open, he won his next tournament (1988 Palermo) while he was still riding the high of being number 1 and holding 3 of the majors. Once that high wore off, the tank was empty.
Wilander was similar to Newcombe in the sense that they both struggled with the day-to-day motivation throughout a long period of time, but they would bring it when they really wanted to.