In this picture you can clearly see that voles were already living in the native area where they had good cover, but while the snow was in place they were able to venture out into the bluegrass. In the path they choose to eat they decimate the available plant but they stay at the soil level and do not go underground. They will continue to graze further as long as they feel safe under snow cover.
The initial look during the dormant winter is somewhat surprising and can be alarming to some. But since bluegrass has a very strong lateral growth habit, these injuries will heel quickly was the turf comes out of dormancy.