Tragic Accident or Crime Connection

Building collapse

One incident has repercussions that lead to the deaths of the leads' fathers and their separation from each other and their families. 

Bus crash

Murders

[major spoilers] The ML's mother and FL's parents were part of a cult when the ML and FL were young. They are obsessed with their religion and neglect the children. The ML looks after the FL. The ML's father tracks down the ML's mother and kills her along with the FL's parents. The FL's adoptive brother is the first cop on the scene. He kills the ML's father in front of the ML. Not realizing what happened, the FL runs through the room and knocks over a kettle. The ML protects her from the boiling water and they both end up with life-long burn scars. 

Murder

The ML's father was ordered to kill the FL. He accidentally killed her twin brother instead. The ML and FL both saw the ML's father kill the FL's friend to cover up the first murder.

The ML's mother and his unborn sibling died in childbirth. When he and the FL are children, members of the FL's household plot to ensure that the FL's mother dies in childbirth. When the FL desperately tries to get a doctor for her mother while she's in labour (despite all attempts from her household to block her efforts), the ML goes to great lengths to help her get a doctor. Despite the ML and FL's efforts, the FL's mother still dies. 

Murder

Bus crash

Political coup / massacre affecting both of their families

Car accident

Plane crash

Murder

Kidnapping

A bloody royal coup

Murder

Hit-and-run

Car accident (but the fact that they're both connected to the accident, which killed the FL's grandmother and is the catalyst that kicks off the FL's ability to see peoples' life clocks, has basically zero impact on their relationship)

Fire