It only does this with 1st plat doses. Lol, I remember the same thing. It was at night, and all of a sudden..... for no reason, I just started sprinting down an empty street. Then, when I hit the 2nd plat, I literally was stumbling around in my room.
For some reason, the 1st plat seems to have stimulant properties more than anything, with just a touch of dissociative effects. Just guessing here, if it's an SNRI, perhaps the powerful stimulant effects have something to do with norepinephrine. The NMDA receptors are involved in the dissociation of DXM for sure, as someone else said.