I wonder what was the matter.

这句话为什么不需要陈述语序？

可以有两种语序，但意思大相径庭。试比较：

I wonder what was the matter. 我想了解那是什么问题。

=I wonder what was wrong.

I wonder what the matter was. 我不了解那是什么东西。

=I wonder what that thing was.