It was explained on the hoops board--
Sagarin's computer has Pitt #10 in ACC = #50 team nationally. It has zags as #11 team nationally. The Sagarin average scoring difference between a #11 team and a #50 team is only 6.5 points on a neutral site. Apparently, Vegas odds makers were independently in close agreement with Sagarin's computer's prediction and set the opening line at zags by 7. Questions about zags bigs being injured or sick and, although playing, may play sub-par has apparently encouraged heavier betting on Pitt. Since Vegas odds makers goal is to never lose money they progressively lower the predicted point spread to get more money bet on the other side (in this case the zags) so they still make money regardless of which team wins.