Many reasons either you have violated their rules, or you have used it for promotional purposes, or used some offensive words that some reliable members with high rank have reported your answers. So these may be the reasons.
People need to evaluate why they use Yahoo or other sites to begin with... Is it to truly become part of a community, or your own selfish interests? or do you think Yahoo owed you something for posting Q/A???? or maybe yahoo Q/A should be renamed too "Your own personal link farm" as someone has stated above. People like you want to use the site above and beyond what the service was ever intended for, and it was never intended for your own personal selfish interests, then get pissy when banned....