There are quite a few people asking about the cause of the time loop, so let me explain:

The cause is Mengmeng (daughter that died several years ago) and that's revealed with the appearance of her ethereal image in the last episode.  

In Chinese cultural context, tragic deaths can leave the spirit of the deceased not being able to rest, resulting in ghosts, haunting, and other supernatural phenomenon. So even if the precise mechanics is not explained, a supernatural time loop would be reasonable within the lore.  Keep in mind, the show isn't really scifi that's focused on the science, but more supernatural suspense where it becomes apparent that preventing the explosion tragedy is the goal.

In the end, the tragedy had been resolved. The leads not only prevented her parents from multiplying the tragedy by killing more innocents. They also compelled a police investigation that brought Mengmeng's victimizer to justice. Her parents were saved, innocents were saved, and Mengmeng finally got an actual investigation to hold the wrongdoer accountable. So finally her spirit can rest/return. The ethereal image of Mengmeng at the end would be enough of a hint to Chinese audiences aware of the cultural context.

That's the one and only issue I had with this amazing drama, but your explanation solves it thanks!