It could be related to thermal expansion. I can imagine the temperature difference between a clear, early morning and a hot summer afternoon to be close to fifty degrees Kelvin, for metal directly exposed to the sun in Florida. The thermal expansion coefficient of steel is roughly 10^-5 K^-1, which translates to a 0.05 % linear expansion over a temperature difference of 50 degrees. Or half a meter per kilometer of track, if you will. The ride track is expanding and contracting several decimeters every day, all year, while subject to the usual wear and tear from multi-ton coaster trains running along the relatively narrow steel tubes that constitute the rails. It's understandable that some fractures appear after decades of operation.
Another big question, I guess, is how long Kumba and Montu have left. Those are, respectively, six and three years older than Hulk and Dragon Challenge (which are of the same age). BGT's Python lasted for 30 years in the Florida sun, a milestone Kumba will hit in 2023. As far as I can tell, Kumba (1993) and Montu (1996) are the two oldest steel coasters in Florida taller than 30 metres (100 feet). They are followed by Dragon Challenge (1999), Kraken (2000) and Sheikra (2005). Given that they're all B&Ms of roughly the same size, and therefore should be subject to roughly the same operating conditions, I'd say that any worries Universal Studios might have had about Hulk should be valid for the other top 5 coasters as well, already or in the near future.