Generally sink holes like what is affecting the spillway are caused by rain and flooding. So I'd say it is likely that the inspections all came up clear for the past X years, then the rain started, the sinkhole formed and the reservoir started filling at the same time.
What's interesting is that you know this. Which means that others should as well. Which means that, I don't know, perhaps modelling what might happen as rains returned, even conducting some tests with tankers full of water could be considered a prudent idea.
In other words, the idea that things could go bad once lots of water came back isn't a far-fetched concept. Yet we are reactionary. We don't seem to spend any time a-priory to try and get ready for such events. History is full of such fuck-ups.
Like I said in my other response; it comes down to money. If we took every "what-if" into account, we'd be wasting trillions of dollars each year on hypothetical scenarios.
This was the first time the spillway was EVER used in its existence; for all we knew at the time, it would have never been used.
