Terrible incident and from what I've seen it's a miracle no one died. This was a very old rollercoaster from a manufacturer that went out of business years ago and which never had a reputation for sterling build quality in the first place.
Whether it be age, rust, metal fatigue, lack of maintenance etc, it proves that though statistically safe, rollercoasters need to be inspected, maintained and have parts replaced, ideally with original manufacturer parts on a regular basis. Not all small parks can afford this and even the biggest operators must be held to the most rigorous of safety audits as airlines are.
I'm no expert but I'm wondering if this was a failure of the bogies / wheel asemmbalies as I note that several sets have landed away from the other wreckage.
Hoping that all involved make a full recovery.