why are the leaves yellow??

[cacoonkitty]

Hi guys ,
please can someone help me , i have recently bought a dendrobium aberrance mini orchid and i have mounted in on a piece of thick grape vine wood to go in my reptile terranium, i have used spagnum moss to surround the roots and have tied the plant on gently to hold it in place with a little moss to pad the plant from the twine, the temperature i am keeping it in is around 79-80 degrees F with a drop at nigh by around ten degrees and high humidity at around 70% created by an external humidifier gently blowing into the terranium every hou or so for ashort time, i mist spray the plant in the morning and at nighg with filterd water and i have fed it witha diluted orchid grow feed, thne thing is i have noticed two of the leaves have turned bright yellow, the other leaves are fine so far and are dark green, is it ok ????why have they done this, as i really like this plant and dont want it to die...am i doing anything wrong??