从深层结构分析，be due to 中的due是形容词作表语，意思是“有原因的”，to 是介词，表示“对…而言”，整体的意思是“对…而言是有原因的”，即“应该归于…原因”或“由于…的原因”。
due to 可以整体看作一个复合介词，后面应该跟名词性的宾语。因为除了about、on、but 和except少数几个介词外，其他介词后是不能跟that 引导的宾语从句的，due to 后只能跟名词、动名词或 wh- 式的连接词。
基于此，The game's postponement was due to that it was raining 是错误的，应该改用第二个句子表述——
The game's postponement was due to the fact that it was raining.
其中the fact 是due to 的宾语，that it was raining 从句是对fact 作出的具体说明，that 在句中不作任何语法成分，只起到引导同位语从句的作用。
同理，下面的句子也是同样的结构，that 从句是对fact 进行具体说明的同位语从句，解释成定语从句是错误的。
His success was due to the fact that he had been working hard.
引导名词性从句的 what 相当于“先行词 + 关系代词”，类似汉语的“所...的（东西/事情/话语）” 。例如：
It is due to what has just happened = It is due to someting that has justhappened.
说明 what（= something that）中有名词的含义，其他如 how = the way that，when = the time that，where = the place that，它们都含有名词的含义。因此可以跟在 due to 后作宾语。
顺便指出，just now 尽管指“刚刚”，但是属于业已过去的时间，因此不能用完成时态连用，不能说 It is due to what has happened just now，只能说——
It is due to what happened just now.
It is due to what has just happened.
