It used to happen to me too, when i was still new to meditation. But well i cant say its normal...or even good(safe) for that matter.
Forexample i go to school...en going into a trance involuntarily is the last thing i would wish for...supposedly am in the middle of a lecture orworse..an exam. A trance is techinically like blacking out... I dnt know for yr situation....probably you need to elaboratemore on how it happens.. In my case they were psyhic attacks...mostsly...and it usually happened when i was alone..doing nothing in particular and my mind wondering aimlessly...a thing used to force me into a nightmare slumber...or put me in a trance...
