Linked events, the occurrence of one, which causes the other, and when the second occurs, it in turn causes the first to happen, and so on.
This is the original definition for this occurrence, the name vicious cycle
is used incorrectly by a lot of people.
He was so hungover he got drunk, and this caused him to be hungover again, so he got drunk again. The vicious circle of drinking continued until he realised he was a raging alcoholic.