(CNN)The fatal cougar attack in Washington state over the weekend isn't just rare; it's basically unheard of.
Cougars almost never attack humans. But here are possible reasons this one did.
The cougar that stalked two mountain bikers and then killed one in the Cascade Mountains on Saturday was just the second fatal cougar attack in the state in 100 years.
But several factors may explain why this attack occurred.
Wildlife expert Jeff Corwin and Animal Planet's big cat expert Dave Salmoni offered a number of possibilities for why this cougar could have tracked and attacked the bikers.
"This is not common behavior that I as an expert can explain away and say this is what is happening. This is that 1 in a million that we talk about," Salmoni said. "In this case, my heart goes out to the victims first, but my next thought is, what would have gone wrong?"
The male cougar weighed about 100 pounds, officials with the Washington Department of Fish & Wildlife police said. That's significantly less than what male cougars typically weigh, experts said.
And hunger and desperation make an animal behave differently.
"This animal was emaciated. It weighed 100 pounds. I believe it was compromised, it was pushed to the very fringe of survival," Corwin said.
"I believe this cat was desperate, it was sick, it probably had a number of injuries it was dealing with," he continued. "As these animals become malnourished, they go for the easiest opportunity for prey."
Salmoni similarly said that the cougar might have been injured or sick, "which would then cause it to start acting in a way it shouldn't."
Fish and wildlife experts euthanized the animal, and a necropsy will be performed to try to determine what provoked the attack, including any disease.
Generally, desperately hungry cougars might attack livestock, sheep or cows rather than humans. But this cougar may have reacted to the humans because of its natural instincts, or what Corwin called the "yarn ball response."
"Cougars are ambush predators," he said. "These guys go flashing by on their bikes at an extreme speed, maybe 20 miles an hour. This animal goes into predatory mode; it's willing to take that risk; it goes for low-hanging fruit."
The two bikers initially fought off the cougar. Isaac Sederbaum, who survived the attack, told authorities he hit the cougar in the head with his mountain bike, and the animal ran into the woods. But as he and bike partner S.J. Brooks caught their breath, the animal returned and attacked again.
Salmoni said it's common for predators such as cougars to fail on a first attempt, retreat into the woods and then wait for another opportunity to attack when the prey is more vulnerable.
"When the prey starts moving off, thinking that they're safe, start showing their back again, start showing those vulnerable positions, the cat might say here's my second opportunity," Salmoni said.
"So when the people thought the attack was over, they didn't realize this is probably more predatory, meaning it was emaciated and hungry and that cat was still in the hunt and unfortunately continued to follow through."
Both Salmoni and Corwin said the cyclists initially did what they were supposed to do when approached by a cougar -- stand your ground and fight.
However, once they caught their breath and got set to pedal away, the animal returned. The cougar fastened its mouth on Sederbaum's head, crunching down and shaking him side to side like prey, King County Sheriff's Sgt. Ryan Abbott said. Sederbaum managed to get loose when the cougar started chasing Brooks, who was running away, Abbott said.
That was an ultimately fatal mistake, the experts said.
"You never run. That's what you never do," Corwin said. "That instigates the bully response. That's the fight response for the cougar. You stand your ground, you hold your ground. You never turn away from the cougar. You look at it face to face, look larger than life, which they did with their bicycles."
"... As one ... was being attacked, the other guy ran away; it was that yarn ball response, this cougar moved in."
Salmoni agreed that running away caused an instinctual response.
"You're exactly triggering instincts. In this particular case, the cat was already hunting so those instincts were already keyed up. But it's absolutely showing yourself as vulnerable," he said.
He recommended that people carry bear spray or bear bangers, which are loud flares intended to scare bears or other predators away.
Salmoni theorized that perhaps the cougar was not raised by a parent. In that case, the younger male cougar might not have been taught by an adult how to hunt or what is and isn't cougar prey.
"In that case, they live off dogs and cats and upgrade to baby cows and continue to upgrade, and in often cases might decide to chase a human being," Salmoni said.
That hunger and desperation may have been exacerbated by the competition between other male cougars, Corwin said.
Male cougars have huge habitats that can have a range of 400 miles, he said. If this cougar was not able to compete with other male cougars, he may have been pushed even further into desperation for new prey.
"The No. 1 enemy for an American mountain lion or cougar is a competitive lion," he said. "This animal was pushed to the edge."