Metal fatigue has been a subject for jets for almost 50 years -- there is an ol' Jimmy Stewart film with Marlene Dietrich about this subject (it is the center of the plot of the movie).
The F-15's have a titanium frame. I imagine that since this is a high performance jet, 30 years of service is too much for the original structure.
If the jet was built totally out of titanium, it might not have this problem (the SR-71 is supposedly an aircraft that is made substantially of titanium, for example).
High performance jets can stress their airframes...
No Highway in the Sky - 1951. Perhaps Stewart's best work.