It is well-known, or better said, well-accepted, that the ancestral language Proto-Indo-European (PIE) was a OV language with a very limited (or nonexistent) use of subordinate clauses. In Proto-Germanic, subordinates was more common (but not too much) but yet a OV language. However, both Old English (OE) and Old High German (OHG) had a VO structure in main clauses, and ancestral OV in subordinate clauses:
|-> OE (VO/OV) -> English (VO/VO) PIE (OV) --> PGmc (OV/OV) --> ¿?(VO/OV)-->| |-> OHG (VO/OV) -> German (V2)
If I'm right, a possible source of shift OV -> VO (in main clauses) is the original topicalization of the verb in PIE/PGmc. In PIE/PGmc the verb occupied the first (or second) position in the sentence to mark some special type of sentences, for example, to mark imperative sentences. This original topicalization of the verb could have became more frequent even changing completely the sentence word pattern.
Quaestio 1: What process created the appearance of subordinates and subsequently how and when did it change the structure in main clauses? And why were only main clauses affected?
Quaestio 2: What process caused English to be strict SVO and German to be strict V2?
Quaestio 3: Was OE really a V2 language like German, or only a left-peripherical language? (V in first or second position, that means, a not strict version of V2).