I was at the game and I didn't really see what they were talking about. Maybe folks watching it on tv saw things you don't see from a stadium seat. But here's my take:

UVA fans are very frustrated that they lost a game to a ranked team in a game that they basically won statistically. So what better way to express outrage then to label BYU as "dirty". Makes them feel better about the loss. The fact that we are an LDS school feeds into the simple narrative that we are self righteous, hypocritical, dirty, etc.