Event 2 has a causal relationship with Event 1 when

Event 1 makes Event 2 happen

Event 2 has a casual relationship with Event 1 when Event 1 makes Event 2 happen.

Explanation:

In other words, a casual relationship between events exists if one of them causes the other. You might understand this better in a term you know, cause-and-effect. In this case, event 1 is the cause and event 2 is the effect. In contrast, a non-casual relationship would be caused when one event did not directly impact the other (even though they might be related).