It is one of the common questions asked about Yoga, but Yoga has nothing to do with religion. When Yoga was founded, the word Hinduism did not exist. It was just culture in India, as there are other cultures worldwide. Some people fear that Hinduism would become popular because of Yoga. Hence, they associate it with Hinduism.
Another reason for this misconception is that Om is chanted during meditation and is the main symbol of Hinduism. The answer is the same as above. The Om was the sound heard at the time of the creation of the universe. Hence, it is universal.
A majority of the non-Hindu population of the world does Yoga regularly. Doctors also prescribe Yoga to the patients. It got momentum in India when non-Hindus started doing it.