Oh god, I actually don't know why this happens.
First of all, I think there is a big difference between fear and mental blocks.
For example on beam, I am sometimes scared to do something because I haven't done it many times before, this is fear.
But mental blocks are different. It happens with skills you were never afraid of before and you did them for months and sometimes years!
I had an experience which was so weird: I was doing standing back tucks for months, I have done a few alone already, with my coach just standing by my side but not spotting. And when he spotted, I he did not help much.
However, in one training session we worked on standing back tucks again. I did around 20-25 already, most of them alone. Suddenly, from one second to another, I got so scared and thought I just can't do them anymore! I really wanted to, I knew what I had to do, but I just couldn't. My coach forced me to and spotted me, but I jumped back instead of up, I arched and didn't tuck at all - it was horrible. Then I was scared if I would try again and them up also, I would be even more scared.
But my coach is really strict and said I need to fight against my brain.
I did some on Tumbl Trak and Air Mat and was fine, although a little scared also. But then my coach forced me to do them on floor again. They were not the best - but I did them. That's the method I got over all my mental blocks I actually only had in back tumbling with RO BH and standing Back Tucks!
I do not know if pressure works for all kids - but I think without my coach I wouldn't have done them for weeks.