Why is this one yellowing?

When I see yellowing leaves in Phalaenopsis, the first thing that leaps to my mind is that the plant has suffered a grievous loss of root tissue. Why this is so? The leaf surface of plants is directly related to their capacity to gather water and nutrients. This is normally done by the roots. if there isn't enough root tissue to absorb the water and nutrients the plant needs to maintain the leaves, the plants usually discard the leaves and grow smaller ones, more proportional to the size of the root mass. But if too much root tissue is lost, the remaining roots may not be enough to gather the resources needed to grow new leaves. In that case the orchid will leave this mortal coil and relocate the to great and happy terracota pot in the sky.

 

It's dying. As to why, I couldn't tell you. You should look up information about growing Phalaenopsis and try to figure out if any of its treatment and environmental conditions are significantly off track. Otherwise, a rapid decline like this could be due to extreme shock, but without knowing what the plant's been through, it's hard to say what caused it. Could it have been exposed to cold temperatures? Very hot temperatures? Did it get exposed to chemicals? A natural gas leak? Did it get fed with too much fertilizer? These are the kinds of questions you'll want to consider if you really want to figure out what happened.

I don't know that we have enough information to say whether you over watered or not, but it's doubtful. If the plant started out healthy, and you've only had it a week, and it's been in otherwise good care, over watering isn't likely to cause this level of decline unless there were clear signs of infection (like crown rot, which isn't evident here).

One thing I think I do see is a white crust in the crown. That could be a sign that the plant is infested with mealy bugs. So perhaps they've taken a toll on the plant while it's been trying to adapt to new growing conditions? On the flip side, if that white crust is due to mineral deposits, then that's an issue because that suggests the water quality is poor or the plant is being feed way too much. Additionally, it suggests that water/fertilizer are getting into the crown of the plant which can lead to a lot of problems (like the aforementioned crown rot).