Whatever actually happened, people have a pretty hard time with random chance. They need someone to blame, and a good reason as to why something bad can't happen to them. If the reason is "it just happened randomly", that's uncomfortable. There's nothing you can do to fix the problem, you just have to accept that every 100 years there's a 0.3% probability that you'll die from some random incurable disease. But if the reason is "some scientist was careless", then they can put that scientist in prison and society is saved from all future pandemics, and they can stop worrying. Of course, that won't actually fix the problem, but it doesn't need to, it just needs to make people feel better.

Especially for claims like this, they're nearly impossible to conclusively disprove, so people will cling to it regardless of whether or not it's true. You'll see this come up again and again forever.