[Lack of exactitude for the sake of one renders it difficult to give any answer that deserves confidence.]

They often trigger in danger or when performing actions. It is difficult to tell what may cause one until it has happened... Once I was being taught how to use a sword, and I had one. A different sword appeared in my hand. [His go-to example from experience, with a moment's hesitation before the last sentence - has no idea if he's treading anywhere nearly lightly enough.]