Vandas too many yellow leaves *(

[P Croft]

I have a problem with my Vandas. I grow in an under the house greenhouse, under lights, with heat, watering from overhead. It seems as though each year when we transition from winter to spring my Vandas loose leaves. As you can see some plants look like plam trees. It is the smaller Vandas,
that suffer worst, and this year it seems worse, I am removing a dozen leaves aweek. I am wondering if when we open the walls as the weather is warmer?? or are the thrips coming in or too much temp change, virus? I would love any help!!