where's the foul

[Texoma_LJ]

1ST year official and after watching the college games on saturday, the following question came to mind.
Runner crosses the neutral zone and is grapsed by more than one player of B. The pile up continues as the pile moves slowly downfield (forward progress not stopped). Lineman of A comes charging into the back of the pile and pile moes forward 2-3 yards and everyone goes down. Rather a common sight during any game. I am wondering why there is never a flag for either (1) assisting the runner by lineman of A. Or (2) blocking in the back by (A).
Seems to me of all the times I see this, there is never a flag for either.
NFHS(preferred)/NCAA rulings are welcome.