I’m kind of thinking two things which are not exclusive:
Turkey shoots down Russian plane to sabotage greater Franco-Russian collaboration in Syria, which undermines their little shitshow on the ground with the terrori-I mean rebels.
Russia intentionally violates Turkish airspace in order to force some kind of rupture in the already crumbling NATO over this issue and the fact that Turkey’s popularity in the alliance is plummeting largely over Syria-Iraq issues.
Both could be wrong, but one or both could be right.
The biggest problem is that the most powerful individuals in each of these nations are both clever but also dangerously ambitious and beholden to terrifying domestic (and in Turkey’s case international) constituencies. A crisis which takes a life of its own is the last thing anyone wants even if the cause is simple pilot error. It probably was, but considering the convoluted nature of everything going on there now, let’s keep our eyes open.
One does wonder what the South Caucasus equivalent to Plevna would be…