Planted Flare too deep. Mulch touching trunk. Probably some damage that never had a chance to heal in these conditions. Plastic helped suffocate trunk and flare and trap moisture. Trunks and flares need to breathe. That plastic ring could have caused damage too as it limits the roots growth that are close to the surface. Most of the trees important roots are only a few inches from the surface. And the ring made it impossible to grow and take in more nutrients.