The cause of clubfoot is not really known. There are a number of factors that are known to increase the risk of a clubfoot, but the actual cause is not that clear.
The appearance of the clubfoot at birth in which the foot is plantarflexed, inverted and adducted resemble the position of the foot much earlier during fetal development. From that early position the foot develops to the more usual plantigrade position that it normally is in at birth. It is assumed that something happens during that normal development and the foot remains in that position it was in during the early development and is born with the foot in that clubfoot position. It is not known what might cause that normal development from progressing.
The most common and widely accepted theory for this is that it is a combination of genetic and environmental factors. There is a strong family history in those who develop a clubfoot indicating that here is a genetic element. The environment factors that might be involved could be speculated to be things like alcohol consumption, chemical exposure or trauma.