Do a search on this forum or google for Female ejaculation or squirting pussies and you will understand why this happens.
Sometimes it is real, especially when it is a jet like you described but sometimes in porn it is faked by injecting some liquid in the vagina and forcing it out with a kegel (squeezing vaginal muscle)
Out of interest, the women who do this are called femmes fontaines (female fountains) in French!