Avoiding over-mixing when using a stand mixer
It could be a lot of things. Cookies in a stand mixer can suffer from gluten production and there would be no really good way to reverse this effect. However, if you are not mixing the cookies for more than a couple minutes, this is not likely the issue. Other things can effect the texture of cookies that might be a byproduct and not a direct result of the stand mixer. 

One example might be that the solid fat that is used in the recipe was over mixed or out too long. Soft fat will make a denser crisper cookie. Refrigerating the cookies for a while would help fix this issue.

Another cause could be a minor substitution. See the blog link below for how different substitutions affect a cookie recipe.

http://www.handletheheat.com/the-ultimate-guide-to-chocolate-chip-cookies/

Along with the above make sure to check things like the expiration date on your baking powder. They lose potency pretty quickly after expiration. This could also cause dense cookies.