Well, if their parents did something a certain way, they might do the same thing, or stay away from doing that same thing to their children because they knew what it was like to deal with that.
Parenting books can also influence parenting skills. Most new parents will go out and research online, or through books to find out what they need.
Family, friends, and the people you're around can also impact what you do as a parent. If you see someone do something one way, why not do the same?
Commercials and infomercials can impact this as well. Buy a certain baby product, and you can be the "perfect parent". But to be honest, who is?
Hope this helps!
Happy studying,
~Mistermistyeyed.