A strange action that takes place is something that shouldn't have happened (but it did). This is a strange incident. The difference between this and 事変 is that while 事変 can also be used to describe a strange incident, it also can be used to talk about another type of incident as well. For example, the "Manshuu Jihen" is the "Manshuu Incident." This word, 変事 is more just a strange or unusual incident.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
Because of the sudden, strange incident, today's stock market is in a grave situation.
All WaniKani users were worried that some unusual incident had befallen Tofugu.
I always get excited during that ominous silence that happens before an unusual incident.