Is Drake possessed or something?

No I don't think so now if you recall this form had another personality when it first appeared. My guess is that it never really went away after Drake gained full control over this form. Perhaps when Drake is pushed far beyond his limits while in his transformed state that dark persona assumes control. Almost like it's some sort of berserker mode that's what I'm thinking here, . Though I could be wrong in my assessment on the matter naturally.

I remember that his alt form has it's own persona, I'm asking if something other than him/it has decided to join the fun, but I see your point, can be hard to tell where Drake ends and the amulet guardian begins in that form.

You got it more correct, except it's pretty much the next stage of Drake's true form.
His appearance and power will increase. :D

Talk about hitting the nail on the head, to be honest I had a theory that his transformed state would evolve the more he used it.