Some people know from an early stage that they want to home educate their child. They may have philosophical or religious reasons for this. Or they may have been home educated themselves or raised in situations in which education wasn't focused on a traditional school environment - their parents may have travelled a lot.
In other cases a child may start off in school but later be taken out and educated at home. The reasons for this vary - a child may have special needs or be unhappy at school in some way. Sometimes parents feel that the methods of teaching in school aren't right for their child and that they can provide a better education for them at home.
Parents sometimes home educate because they can't get a place for their child in a school of their choice.