I wrote here
I’d agree that fear […] can be infectious, unfounded, paralyzing/stampeding, and difficult to resist. And sometimes that’s exactly why it’s a useful instinct sending a signal that you need to prepare yourself for trouble.
For example, if the stampede is starting, you’d better get ready to run, or you’re going to get trampled. Just look at a chart of the DJIA for the last 6 months.