Metatarsus adductus is a mild deformity of the foot present at birth in which the forefoot is angled inwards on the rearfoot. The cause of metatarsus adductus is not known or not clear. Some kids are just born with it and some aren’t. Around 10% of babies are born with it.
However, there are some theories and one of the more plausible is that metatarsus adductus is the most mildest form of a clubfoot. A clubfoot is typically when the foot is angled downwards and inwards at birth. This is a position that the foot is in early in fetal development. As the fetus develops the foot moves towards its more normal position that it is in at birth. In those with a clubfoot, something happens during the pregnancy and that foot stops it normal development.The reason why it stops is not clear, but could be a something such as a minor problem during the pregnancy (eg drinking alcohol) or something like that. The same could be an issue with metatarsus adductus as the position that this is in, is a normal position of the foot earlier in development. So is is assumed that something happened during that normal development from the adducted position of the forefoot that halted the development. It is not clear what they might be, but there are a lot of speculated reasons.