Russia is saying they are moving S-400 sams to their base in Syria (for some info, here). They will then be in a position to have Assad announce that Syria is no longer allowing uninvited foreign aircraft into their airspace. And they will have the means to enforce it. American ability/resolve will then come into question a bit more, pushing America to make their obligatory tit-for-tat.
All the logic about why they wouldn't engage each other (east vs west) is good, but sometimes events take a course that neither side intended. I see them moving in a way that reminds me of how WWI got started. I wonder if ISIS might not like the idea of propagating an east-west war as a means further their own caliphate.
Bobcat