Chances are something got left between the display and the base of the laptop. Since the clearances are so narrow it wouldn't take much to cause a crack. Even a business card could do this if a card became lodged here as the lid was closed. It is unusual to see the crack in the center of the bezel. Most bezel cracks I've seen have been on the left or right sides (within an inch or two of the edges).
You can try putting some black electrical tape over that cracked portion of the bezel to try to keep it from falling apart. You will probably need to trim the tape a bit to fit within that bezel area. There are very fragile & delicate cables located beneath that bezel though. At this point it doesn't matter since Apple will consider this accidental damage regardless if it gets worse.
I have replaced the cracked bezels on our organization's Macs, but it should only be done by an expert since it is difficult to remove the broken bezel without damaging the delicate & easily damaged cables or melting the rubber & plastics. The replacement bezels are ones which have been pulled from other laptops and sometimes the adhesive on them is not the best (although so far it has been adequate). It can be difficult to align the replacement bezel as well. Trust me this job isn't for the faint of heart. The bezel is inexpensive, but the time involved is a lot so there would be a decent labor charge.
If you have Apple replace the Display, then you may want to get a repair quote from an Apple Authorized Service Provider since their price should be significantly cheaper than an Apple mail-in repair for accidental damage.
