If you only planted it two months ago, the most likely cause is a problem with water supply, as in not enough of it. Do not fertilize the tree at all while its in a critical state, but you should soak it daily with at least 4 gallons a day, or leave a hose trickling at the base of the tree for a couple of hours. Do that even if its raining, unless the rain is absolutely torrential and goes on for a day or two. New trees need copious water supplies for their first two years, and particularly so if you plant in spring and not fall.

The other possibility is whether the tree needed hardening off prior to being planted out - if it was packaged in plastic, or had been kept under shelter or inside, then it would have done.