To give Disney World management the benefit of the doubt, a lot of people have speculated that Florida's much, much wetter climate may have something to do with the structural problems of the Magic Kingdom's Splash Mountain.
The animatronics, sound system, and fountains though? No excuse.
I kind of doubt it. First of all, there are ways to protect concrete, and the rebar set inside the concrete, from moisture. Splash Mountain in Disneyland is exposed to moisture as well, quite a bit actually as the ride does use water and it does drizzle quite a bit in SoCal. Secondly, they knew going in about the climate in central Florida.
Things like having a long concrete curing time, or making sure the rebar depth in the concrete is deep enough (by using more concrete), all seem to be important. Possibly Disney is using different fibers/rebar for new rockwork (Carsland), but Splash in MK isn't that old when you look at how long such structures can last, which is centuries and more if taken care of properly.
I would guess that proper inspection and upkeep did not occur.
A limb fell off the Tree of Life, most likely corrosion, but we've never seen scaffolds go around the tree for an inspection and refurb, in all likelihood, it was preventable.
I think MK just doesn't receive adequate maintenance beyond animatronics, things that should last forever aren't properly protected from corrosion. I would think that paint, and other protectors, would need to be inspected and surfaces recoated as needed. Doesn't seem like this happened.
The goal with a roof, with anything exposed to the elements is to keep water out. Somewhere along the line, there was a failure in some coating, or with a specific type of concrete mix used, which would go back to when the thing was built.