Benteke did nothing in particular, he braced for someone to run into him, Sagna had no god given right to jump through Benteke, that was basically always going to be the outcome, Sagna was never in a million years going to outmuscle Benteke, come through him and win the ball.
The landing was just unlucky, wrong height jump, wrong point of contact and boom, horrendous fall, he could still easily break something, as in he walked away from it but that could be cracked vertebrae, anything really, horrible fall, lucky the ground had some bounce in it. But falling like that he could have broken his neck then and there, I assume/hope nothing serious happens. Sometimes with neck injuries the damage is there but the swelling takes a bit of time.
He's had this thing about jumping into people all preseason, he smashed into two players the same way in the Emirates tourney, they just weren't built like Benteke so Sagna and the other player landed in a heap. I wish in particular our injury prone players would use the tiniest amount of common sense and not run around looking for trouble.
After that hit I hope he's rested completely for a few days, sent for a scan, triple check everything is okay and not rush him back, you don't want to mess around with a neck/back injury and Arsenal have a habit of underestimating the severity of things just to find a injury get worse and cause serious issue(RVP and Wilshire for instance).