Shamu The Killer Whale Attack?

SeaWorld in San Diego has canceled its Shamu whale show at least for today because an Orlando SeaWorld employee was killed in an encounter with a whale at that theme park.
The killer whale is called Tilikum and the trainer Dawn Brancheau.
It was an unexpected attack since the trainer knew the killer whale for years and never had a problem.
Why did Shamu the killer whale attack the trainer in Sea World ? Was the whale angry for some reason?

Eyewitnesses told local television stations that Shamu the 12,000-pound killer whale, which the female trainer had just introduced as a public show started, came out of the water and grabbed the woman by the waist and shook her violently.

After this the trainer slipped into the Shamu tank and drowned to the shock and horror of the guests. Sources say the whale "took off fast in the tank, and then turned came back, jumping into the air, and grabbed the trainer around the waist. The whale then began shaking the trainer in its jaws."

Brancheau was pronounced dead at the park after being recovered from the pool. Orange County Sheriff's Office homicide investigators continue to investigate the death of the trainer who was reported to have 16 years of experience working with killer whales.

Ms Biniak said the attack was violent enough for Ms Brancheau's shoe to fly off.
Brazilian tourist Joao Lucia De Costa Sobrinho and his girlfriend told the Orlando Sentinel they were at an underwater viewing area when they saw a whale with a person in its mouth.