There is a thing called over sheeting where you increase the angle of attack so much that the kite losses power/ disturbs the air flow over the kite. In this case the kite often flys backwards

Is the kite new, as after the first couple of sessions the front lines stretch a bit

Don'T forget that angle of attack changes as you fly the kite through the window.

Next time you drive your car stick right hand out the window make it into the shape of a wing(more or less)start with your fingers parallel to the ground, and slowly twist your wrist up till your thumb is pointing straight up you should feel the effect of angle of attack. Please only do this when it is Safe, no other cars around ect.. I'd hate to hear you lost a hand trying this