How do side surround sound speakers simulate sound coming from behind you ?

Not sure. I think they're paired with audio that's divided into channels, just like your monitor speakers would be with Pan 1 and Pan 2 for Left and Right front side sound. I think there's a home theater control unit that distributes the sound accordingly to the appropriate surround speaker. Creating the illusion of live action. The audio track is usually encoded, I believe, telling the master controller how to distribute the sound.

I could be way off though. Google it.
It's been years since I read up on it, back when it first debuted.

That is a very good question... And Google has the answer.

Because..science.