Why, because there is the train of thought that the ACC, playing second fiddle in most states, is in a very precarious position. The lack of money while stuck in a contract that will pay less than every other conference, up to $25million less per team than their direct competitors, The Big10 and The SEC. That's until 2025. You pride your olympic sports programs, but they cost money. What happens if the proposals from The Big10, SEC, and PAC12 go through? How will you afford to support those programs? Wasn't there just a panel of ACC ADs talking about how they weren't going to be able to afford the proposed changes? UVA is not financial trouble right now, but the university has to decide how they are going to proceed. If The ACC continues to lag behind as far as they are projected to, it will make a difference in how go about their academic business. The changes may not take place, but the finances of their competitors will. This doesn't include the exposure aspect that the school loses having most of their football games on ESPN2 and ESPNU outside of Virginia (if at all).
UVA administration may be very happy with that. They may want to downgrade to an Ivy league like status and I would never berate them for it. They may de-emphasize their football and/or basketball programs. They may have to subsidize form their general fund. They may not have to do anything. Just like you, we are all speculating on what will happen.
Whether true or not, there were many rumors that UVA was in talks with the Big10. They fit the profile of a Big10 school (large, flagship, highly rated research university) and fit the population demographic well, too. The popular belief is that UVA would never want to go to The SEC and, if they went to another major conference, it would be The Big10.
I completely understand why you say the fans are happy. I'm sure most are. There are
some that have big issues with the direction the ACC is going on a few aspects, but my guess is that over 90% of the alumni are happy with the ACC. Unfortunately, what the fans and alumni want isn't always what is best for the school. College football is what it is because of tradition. Most fans don't follow a school because of a player, they follow the school. Either it's a link to the place where their liver were transformed, or great times they had there. I wish that Michigan would play for the Little Brown Jug every year. I wish that we could play Illinois (don't ask me, I can't explain) more often. In today's world, it's not happening.
Now, you asked why? These are my reasons. I have no clue that The Big10 is expanding. I think that they are and UVA is one they like.