I've worked in a few coffee shops. Why and how did the term "barista" come to be applied regardless of gender (as opposed to baristo)? What is the history of the word?

History of words is etymology; you can find at least some etymology in most dictionaries. The Online Etymology Dictionary is a handy online reference that says "barista" came to English from Italian, where it was also genderless. To get more details on the term or its history in Italian, maybe try an Italian etymology dictionary or Italian linguistics/language sub

barista (n.)

"bartender in a coffee shop," as a purely English word in use by 1992, from Italian, where it is said to derive ultimately from the English bar (n.2), as borrowed into Italian. The word is of generic gender and may be applied with equal accuracy to women and men (it is said that the typical barista in Italy is a man).

http://etymonline.com/index.php?allowed_in_frame=0&search=barista&searchmode=none