Ajudge who shares a president's views when first appointed may rule differently on cases later. why

This is because the job of a justice on the supreme court is to deem whether laws are constitutional or not. No matter how much a supreme court justice supports a president, if the president's acts are unconstitutional then the justice will point that out and rule against that person. This is because of the check's and balances. Although the executive branch nominates and the legislative approves, the judicial branch monitors their work. The executive branch cannot control the judicial.