Starts when I get to Menethil Harbor. I take crazy flight paths through the buildings there and through several mountains along the way. Hinterlands is pretty bad on that path as well.
I also experienced this same issue. Flightpath from SW to South Shore. It stopped a little ways after Menethil, turned me right around and flew me through a building. Then it took me back through the building, all the way up to the Hinterlands (passing through a mountain on the way), then finally back to SS.
I did this the other day.
I did soooo many loops around every single city in every zone that I almost auto logged out by the time I landed.
My hippo gryph also immediately started to face me as if it were going back to Stormwind multiple times through the flight.
It is unnecessarily long. Too long.