Ny one knows why churro dough poped in the oil ?

Frying churros can be a serious walk on the wild side, as you have seen. There is actually some science behind a successful batch which deals with ideal surface area in proportion to the area of dough inside, and the relative water content of the dough. Put simply, you need more surface area cooking than dough inside steaming. And that steam needs to be able to escape before the outside is set too firm to allow it- or KABOOM. So the ideal piping tip is a small star - usually about 3/8 inch in diameter for home churro purposes. I've also found it handy to fry in small batches, with a splatter screen on top of the pan. That way if one does blow, there is less collateral damage. And if you are going to deep fry things, it's really safest to use a deep fry thermometer, or a purpose made deep fryer device. That is the safest way to play with hot oil.