Bad trips aren't a singular thing, and they don't have a singular cause.

First you have to define what a "bad trip" is. Some people describe any unpleasant experience during a trip as a bad trip. Others reserve the term "bad trip" for a much more profound, severe reaction that isn't merely unpleasant, it's horrific.

The psychedelic experience lends itself to extremes at times. An intrusive thought that would normally be discarded can be obsessed over. Confusion can lead to thought or action "loops". Interactions with other drugs (such as THC) or external events can cause anxiety/paranoia.

In most cases, there's the possibility of a feedback effect. Something is unpleasant, creating a heightened reaction, which makes things seem even more unpleasant, creating an even stronger reaction, etc. This kind of feedback can be difficult to break out of, particularly if you let it get past a certain point.

With high doses of psychedelics, the user's perception of reality can become whatever they think it is. That can be great, but it also means that if you think that horrible stuff is happening, horrible stuff effectively is happening. In extreme cases, the person having the bad trip is immune to logic or external stimuli...they are in a world of their own making. You can't reason them out of it, and they can't either. All of their senses are telling them stuff that isn't true.

Anyone who says that "there is no such thing as a bad trip" or "there's always something good that comes out of a bad trip" or "you just need to relax and go with the flow" has never experienced an extreme Bad Trip.

Reason doesn't always work on people who have lost the ability to reason correctly. Or who are reasoning correctly, but are responding to invalid input.