??

When in a fight for your life the body dumps adrenaline and other hormones/chemicals into your system to keep you hyper alert, focused, energized and all those other things you need to survive. This can often cause very high emotions. These chemicals also shut off some parts of the brain because that part isn't needed for survival and gives more focus to the parts of the brain that is needed. An example is fear. When you have a strong feeling of fear and go into survival mode the parts of your brain that retain long term memory, conduct foresight, and rationality stop working.

So, you take a couple who don't realize yet, but are catching feels for each other, has extremely good chemistry on the battlefield together, saving each other's lives repeatedly, finding you have complete trust in the other, with all those high emotions spiraling around and parts of the brain that tell you to behave not fully working right. I think it would be more strange for them to have not suddenly have a fiery moment of passion.

It does happen rather quickly though, so if you don't catch the subtle hints of a building attraction in the chapter before or didn't think anything of the growing affection because it wasn't enough to cause such a passionate kiss, it can make you feel like you missed something.

They’re battlesexuals. That adrenaline is making them horny for each other

Okay I figured that was the case, and maybe a bit of mistranslation too. haha thanks!