Ergative construction in Middle Indo-Aryan and Iranian

I'm reposting since I didn't get any replies the last time (not
sure it got through either), and I'm sure Piotr or someone knows
the answers :-) thanks, Harald

Hello!
Since we have some experts here on IA and II developments, what is the
status of this common feature ? As far back as sanskrit I've seen the
agent in the instr., patient. in the nom. and past ptcpl. in -ta-
congruent with the patient - completely ancestral to the case of Modern
Hindi. In Indo-Iranian, for instance Sogdian or Pashto it doesn't look
the same morphologically, but completely the same structurally i.e in
transitive past sentences. Are there are theories on how it developed?
Independently in II and IA or joint? Due to some substratum? Why only
in the preterite-ish tense?

thanks in advance

Harald