Woran er nur wieder gedacht hatte

Question

Here is a sentence:

Woran er nur wieder gedacht hatte!

I don't quite understand why this sentence follows the Nebensatz word order pattern. Is this a special type of a sentence?

Answer 1

This construct is called Ausrufesatz, see e. g. here, at leo.org or the grammar book of your choice. In such a sentence the verb may be located at the beginning or end of the sentence.

In your example the sentence (see subsection Ausrufesetz mit Frage in referenced page) expresses astonishment of the person itself, to which thoughts they deviated.