As I recall, Missouri theme park code as it is written now isn't really that old, maybe within the last 20 years or so, and even then, it still isn't as strict as state codes on the coast.
It does seem to smack as they were finding flaws and then added them to a list and started throwing darts at them so that they could fulfill their statutory obligations. When your vehicle goes all day making mostly right-hand turns, it goes without saying that the bearings are not going to wear at the same rate. Likewise, you could tighten the bolts every morning with a cheater bar and then come back at the end of the day and find more loose bolts due to the heating of the day. I would take a list of possible causes like that a little more seriously if they were coming from the likes of the NTSB.
I tend to agree, we did get some heavy rain. Anybody who has lived here awhile knows that when we get a lot of rain, a road somewhere is going to try to move. Sometimes it happens overnight, sometimes it takes weeks to show up. It makes a lot of sense that the road in this case may have been a railroad bed.