[Babul]

it seems to be a bad scripted anti-fall-off-bike feature. check your OnPlayerStateChange() callback, and look out for any SetPlayerPos(playerid ,its obvious that the setplayerpos gets called when you change your player_state at high speeds - to detect that, there shold be some sort of speed-calculation. most prolly using the GetVehicleVelocity() function...
as soon you spotted the line which sets the playerid's position, simply //comment it out, and maybe add a GameTextForPlayer(playerid,"plop",5,1000);