The aircraft, according to the radar tracks the Turks are providing, was only over Turkish airspace for a few minutes at best. So we're not talking about an egregious violation here (egregious being up for interpretation.) The Turks say they warned the craft numerous times including points prior to crossing the border, so they were prepared to shoot it down after seeing it would cross if no course change was taken.
Some might ask what the harm is to Turkey in this case, and my guesses are this:
1. If Turkey didn't take action, then they could set a dangerous precedent by which aircraft were tolerated to violate their airspace, but Turkey would then find it difficult to then crack down after the fact. How deep is a real incursion? At what point do they say "ok, we're really gonna shoot you down now!"
2. The BBC brought up the fact that there are ethnic Turks (Turkmen) living inside Syria, and some of them have been targets of Russian/Syrian bombs. The Turks have been notably irritated at this, and so one could imagine that this gave Turkey an excuse to hit back.
I saw the initial Russian comments as insisting that their plane was downed by a SAM as a real desire to downplay the incident. But now that they're acknowledging it was downed by a Turkish aircraft, it seems they're at least wanting to object harshly. (Edit: That, and Turkish insistence that they did it. Maybe they didn't see the wink?)
I agree with some of the other comments, it's far too early, and even alarmist, to assume that much worse will come up this. A larger conflict is in no one's interests and is in no one's intent.