You are most likely observing a reversion (a shoot reverting to the tree's original characteristics).  Many red-leaved Japanese maples are grafted onto the green species of Acer palmatum. Grafting takes advantage of some of the best characteristics of both types of tree: a strong and hardy root system and a beautiful leaf canopy. When the deer injured the top of your tree and it had to grow back, it probably grew from the portion of the tree that was below the graft line and thus has the characteristics of the green species of maple.

There are other reasons that red-leaved maples can become green-leaved but the injury and response makes them seem less likely. None-the-less, here are some other possibilities.

When Japanese maple leaves lose their red coloring uniformly, it is unlikely to be disease or pest related. Common pests and diseases result in spotted, brown, or desiccated leaves. But there are a number of possible reasons that you notice Japanese maples becoming less red.

• Environmental factors are usually the cause. Too strong sun or, conversely, too much shade can cause color change. In sun, the leaves may scorch or fade to a duller tone. In shade, leaves produce more green chlorophyll to overcome the lack of light.
• Some varieties retain their red color better than others.
• It is common for red-leafed maple to have red leaves in spring that then turn green in summer. Some cultivars change color numerous times during the growing season.
• Too much nitrogen-rich fertilizer applied in the spring. Even if you are not fertilizing the tree directly it may be receiving fertilizer applied to nearby lawns or flower beds.
• Growing conditions are too dry.
• Very hot summers are also a factor and lead to leaf fade.

Courtesy of NYBG Plant Information Service