Pilipino nicknames are often derived from words and phrases of random cultural significance (like some people use the nickname "Nene" to refer to a woman they've known for a long time, which is short for "Neneng" or little girl) or part of their name (Jejomar to "Jojo" for example -- which also happens to be my father's nickname). There's a lot of repetitiveness and playfulness in nickname choosing.
As to exactly why this is the case, I don't think there's any one official theory. This BBC article has a good primer on it, and this blog also gives a pretty convincing linguistics theory as to why:
http://news.bbc.co.uk/2/hi/programmes/from_our_own_correspondent/9435751.stmhttp://askthepinoy.blogspot.com/2011/02/why-are-filipino-nicknames-repetitive.html
