Hello,
first I'd like to say I have only done this a few times and I'm not a good lucid dreamer. Here's my theory:

I've experienced a high correlation between streaming and nightmares
What I mean by streaming: Hypnagogic imagery (hypnagogy is the tired state between being awake and asleep)
It is easily induced, kind of like meditation in that you sit, wait, and observe, but instead of simply accepting all thoughts, with eyes closed you try to receive more of the random images that flick in your imagination. They tend to be very fast and very random. You have to imagine something sometimes, so the stream keeps going, but ultimately it must not be you who is doing it. Eventually it should grow into a feeling where you flow with whatever the mind throws at you.

The few times I did this, either intensely for about an hour or before going to sleep, I got a nightmare the next night. The events in the dream were not good to my wellbeing, I often felt the presence of something else, my body was being stretched, pushed and thrown around without my control, all with an increasing feeling of fear. But it was all because I tried not to resist anything that happens.

My take on explaining it - when you dream and you become lucid, the dream will be built around your attitude, what you want from it, it can be about flying or whatever. But if you happen to recall your attitude from streaming, where you do not resist anything that happens and actually support all suggestions, it kind of breaks.

To test it, take all the control you have over your dream and... lose it? Surrender it to first thing that comes to your mind. Resist nothing. Next thought comes, materialize it. Flow.
I don't know, it's like bringing tantra/meditation into a lucid dream. I say weird things will happen if these two cross.