Maybe in your first language you feel deep inside that you should be on the same level as your peers. Or you experience other reasons that people judge you more in the first language or you have more fluency pressure. If you perceive that you can make errors when speaking the second language, maybe it reduces the fight flight freeze response and then your mouth apparatus likely won't freeze. If, in your first language, you feel the need to cancel the speech plan and repair errors, then you will likely block more, such as if the first language requires more complex syntax, grammar, and phonology, then you likely justify holding back speech. This is my opinion. You likely have completely different reasons to hold back speech though because every person probably has different beliefs/attitudes to freeze speech muscles. [These](https://www.google.com/search?q=%22bilingual%22+%22research%22+%22stuttering%22) researchers explain it better.