I would guess that it has to do with the food near the surface that they're gorging on, some marine animals (e.g. dolphins) do that apparently for fun, and I see no reason why mantas couldn't do that, too.

I entered your question into Google: Why do mantas jump out of the water at night and belly flop back into the water? There's a video link that says:

Mobula-Rays- Belly-Flop-to-Attract-Mates

Here's a link about manta rays, which are larger than mobula rays:

The-Amazing Flying-Mantas-of-the-Sea-of-Cortez

My other answer is the same as the answer I gave a little boy who asked, "Why are snakes so long?" I asked him, "Why do you have 5 fingers?" , and we wondered about that for a while. [We don't know exactly why snakes are so long or why we have 5 fingers.] But here's a better answer: "Snakes are so long, because the long snakes survived better and had more offspring."

So I'll tell you, "Manta rays belly flop because the manta rays that belly flopped survived better and had more offspring than the manta rays that didn't belly flop."