mass- hysteria

What [or whom] sets of mass-hysteria?

To my amazement I saw thousands of -otherwise no-nonsense Dutch- crying and throwing flowers at the funeral of the murdered Pim Fortuyn.

here:

The man was an opportunist and a dangerous right-winger.

How come all these people suddenly find the need to go out on the streets?

Just a WAG, but my feeling is that even those who disagree with Fortuyn’s political views dislike political debates being fought with guns. It’s just not a nice style, so to speak, and even if you’re anti-right wing you might pay tribute to a rightwinger who fell victim to it.