From my experience bad trips/loops are pretty random.For me burning the spice,using too high a dose or using too often seems to up the chance of it happening to me.

I have also been "locked out" a couple times years ago, both times it was from smoking way too often.
I wasn't locked out from having a psychedelic trip but more it was the same negative trip 3 times in a row and the same entity telling me it was always going to be like this from now on.

I took a good long break after that and never had that problem again.

Not saying you cant use dmt often,it just increases the chances of these things happening.