Why the light curve goes down when the planet is behind the star? There is a video explaing the transiting exoplanet light curve — https://www.youtube.com/watch?v=RrusIZaWDW8

It is clear to my why the curve goes down when the planet is between the observer and the star, but I don't understand why the curve goes a little bit down when the the planet is behind the star. I was expected that this should not change the light curve at all.
Think of when the planet is at the "side" - there's a little bit of light from the planet (ie, reflecting off the planet) shining towards us.

Could it be due to that little bit of light - when the planet is behind the star, it no longer reflects towards us?

Maybe that's the effect you have in mind?