What would cause a co-worker you've known for years to leave (for a new job) and say no goodbyes?

Some kind of misunderstanding. If you were friends, someone could have been jealous and lied and said you talked behind her back. Or if you said something someone could have misunderstood what you said. I would call them and ask if they are upset and talk about it. Also share your feelings. An example is " Hi I felt so sad when you left and I hope you are ok?"