There appear to be four main causes:
Lots of eye contact at close range can cause anxiety or trigger a fight response.
Staring at your own face can make you highly critical of yourself.
Being trapped in one spot can make a person restless.
You can't see a person's body language, so it takes more energy to communicate.
"It's a wonderful tool when used properly and sparingly, but ya know, doing it 9 hours a day is not the same as going on a hike with your friends or being at a rock show and just being around people," said Stanford Virtual Communication Lab's Founding Director Jeremy Bailenson.