In the U.S., we seemingly prefer the former to the latter. However, I was sitting with my friends when one of them stated that he was "disorientated" while we were playing a video game.
My theory, at the time, was that he made the language connection between a "youtube game commentator" and "disoriented", resulting in a mash-up of the two words. Due to the rise of usage in the word "commentator", at least in the U.S., it made plenty of sense.
However, I just did some look-up this morning, out of curiosity, and it seems that the British prefer the latter to the former. It seemed a bit odd to me, so I asked some acquaintances in an SO Chat Room, and an interesting proposal was made.
Perhaps "Disoriented" is more of an adjective, where "Disorientated" is more appropriately a verb.
An example would be:
- Adjective: He became disoriented in the forest.
- Verb: He disorientated himself in the forest.