As you say, there has been plenty of notice given to investors...

As you say, there has been plenty of notice given to investors that the company would enter the Index, so there has been an incentive to anybody who wanted to hold stock and release it into the auction on close, given that these events almost always result in prices rising - hedge funds and brokers will have been buying up stock over the past many months and yesterday they achieved a return. There is no guarantee, but the closing auction was fuelled by investors buying up parcels of stock to fill their institutional requirements. Now the auction is closed, the stock is in and life goes on as normal, supported by inclusion in the Index which will rebalance each day.