What causes children to walk on their tip-toes. Is that a sign of autism?

Toe-walking that. persists > 3 months after a tot's first 10 independent steps is atypical. Drops in blood flow & oxygen, especially in late 2nd trimester can cause " tight heel cords", very mild cerebral palsy, more common in preemies. MRI & microscopic post-mortem studies of brains of people with ASD show abnormalities in areas that control motor planning & in axons that transmit signals. Other causes exist.
OLD STORY. Used to be considered sign of autism. But some kids do it for fun. Maybe you did as a child. Don't be in a hurry to make a diagnosis. If you have other reasons to be concerned have the child evaluated by a child psychiatrist or psychologist.
Not necessarily. Toe walking is common before 18-24 months and rare after 3 yo. It is not a sign of autism.
Tip Toes. Infants are born with relatively tight heel cords and initially will tip toe a lot. This slowly changes with time. But some will tip toe into toddlerhood and longer. If your child is older 4-5 you may want your doctor to evaluate. Of course there are other causes but you did not give any history on your child.