I see no evidence of the plant being sunburned. What I see is a plant that I feel is being overwatered. You should have repotted it into the clay pot instead of leaving it in the clear plastic pot.
When you water an orchid and it is inside of a pot and then in another pot, it too often leads to the roots and media not being allowed to dry out properly between waterings. As a consequence the roots towards the center of the pot, and those down deep in the mix are not drying enough. If that continues to happen, you accelerate leaf loss, particularly the lower leaves go first. Those roots rot in the media and " root rot" often develops leading to the death of the orchid.
This double potting issue is something I see often with beginning orchid growers and I see it most often with Oncidiums and Phalaenopsis. My best advice is to grow it either in a clay OR a plastic pot, NOT both!
