There were 3 whales - Tilikum and 2 females (Haida and Nootka, IIRC) - in the pool at the time. They generally didn't have a lot to do, though on rare occasions, they'd be given a toy to play with. To them, anything that got dropped in the pool became a toy, and sadly, Keltie Burns slipped and fell partly into the water. There is apparently some evidence that it was Tilikum who pulled her the rest of the way in, which is why he has gotten most of the blame, but all three of them were involved.