Worst case Tpd value changing

The fact that some timing delays are unstable doesn't mean that your design has problems per se, as long as they are all within your tolerances. The problem is that if you don't constrain your project, Quartus has no idea what kind of timing performance you require, so will try to make some educated guesses. As it uses a pseudo-random algorithm for the fitting part, as Alex just said, it can generate completely different results from even a tiny change in the design. If you need more timing stability, and to be sure that your design will run within your timing requirements, you need to constrain your project.