1. The plane was hijacked (this seems the most likely for now but let's see what develops)
2. The engine data and satellite ping data was erroneous, perhaps confused with another source.
3. You have a slow issue like slow decompression combined with an avionics failure which caused the plane to follow a long and complex course.
Under normal circumstances none of these three are reasonable explanations. For it to be a hijacking, someone has to have been interested not merely in takin the plane somewhere else but in stealing the plane and concealing it. And the other are remarkably complex.
At this point, though, those are the three scenarios we are discussing. The author of the theory suggests that investigators are using bad data. I wouldn't say it is impossible, and we are already talking about the margins of what is possible in all of our possible scenarios.