以前见过这样的句子：

1) We do not know exactly what happened that eliminated so many species in a relatively short period of time, but there are several possible explanations.

还有之前讨论过的句子：

2) What is going on here that is so important to where Florida needed to steal this land from Alabama?

麻烦提供一下先行词 what 此种用法的权威出处，如某权威语法书或惯用法辞书等。

十分感谢。