I'm not sure twitter is the cause, more the primary avenue. People can be dangerously groupthink-y and overreacting in person or on social media, and our culture certainly seems to be moving more in the direction of instant reactions and emotional self-satisfaction over engaging in nuanced debates.

Well, is twitter actually the problem, though? Twitter's biases are a problem, but that's a separate issue. Twitter can't be expected to prevent all forms of propaganda from its users, and even if they could the problem is not just twitter specifically, it's all social media. While the parent made a pithy comment using Twitter as an example, in reality instigators rely on an array of platforms to spread the message and whip up the proverbial mob.

And more importantly, people with decision-making power have shown a remarkable tendency to submit to the will of these capricious and volatile online mobs.

Why is Twitter the problem if a company fires an employee.

If I can just ignore Twitter (and I do) then Twitter mobs don't directly affect me. But if my management does things because of Twitter mobs (or anything else like that) we have a big problem.