More buyers than sellers. There isn't always a good fundamental reason.
Having said that, I think it is just that people are a little less afraid of deflation/believe in some inflation. Inflation is good for borrowers having a hard time making their payments so AGO and others who have been at risk see their potential liabilities decline. Central banks impact share prices far more than people like to acknowledge.
Fairly sure that you are joking but in case you aren't, "more buyers than sellers" is just an expression. If there are more offers to buy than sell at a given price point, the price will adjust upward in a competitive bidding situation.