[JRutledge]

If you feel the tackle was caused by the pulling of the "horse-collar" then you have a foul. If you feel that the use of the other hand contributed more to the tackle, then you could have nothing. Basically it is not a foul unless the action of the "horse-collar" was the reason the runner went backwards. It is hard to tell by your description as it does seem like this is likely a foul, but it does not have to be.

Peace