From the The Conversation article:
Toe walking can be classified into four groups. Some neurological conditions such as cerebral palsy or muscular dystrophy cause the calf muscle to be tighter or change the way it works. This makes it hard or impossible for the child to get their heels onto the ground to walk with their whole foot.
There are also orthopaedic conditions such as congenital talipes equinovarus (also known as club foot) or calcaneal apophysitis(inflammation of the growth plate in the heel) which cause toe walking. The structural change in the foot or leg makes getting the heel to the ground impossible or painful.
However, some children who can get their heels to the ground prefer to walk up on their tip toes. This group of children also has other behavioural characteristics, such as delays in achieving milestones or ritualistic behaviours. Here toe walking is associated with autism spectrum disorders or developmental delay.
Read more at the link:
http://theconversation.com/what-it-means-when-kids-walk-on-their-toes-59081