It's very difficult to say what this speaker means without seeing from the context if they are implying something negative or positive. This isn't a commonly used phrase, but it would probably be meant positively, like something very exciting is going to happen. Like Clare said, it would be more recognizable if they were talking about something like inspiration or success. If it is meant negatively, a more common phrase would be "a disaster/tragedy/nightmare waiting to happen."

I haven't heard this expression in this context before, as in waiting for something bad to happen. In my experience if you are "waiting for lightning to strike" you are waiting for a good idea to come to you, for inspiration.

I think it means that if you have that kind of (negative) attitude, in a relationship. conflict & misunderstanding would happen. Thank you.