It is a fear reaction.
I do 'trance' my bunnies for giving meds and nail clipping but I keep the period of time as short as possible. Although it is uncomfortable for them it minimises the risk of damge to their nails, before I tried this technique I seemed to cut at least one quick everytime when they struggled, even in a towel. Now it is very rare that this happens.
The position I use is to wrap them in a towel, lie them on their backs between my knee's with their head facing me- I used this position for a while before i became aware of what it was and why it works.
For some reason it donst work on Marley, he is impossible to trance so we just have to have a struggle between us.