Players of the kicking team aren't eligible to block until they're eligible to recover the ball, which usually means either the receiving team has touched it or the ball has gone 15 yards. Where the block occurs doesn't matter, so if an onside kick is dribbling along the 40 and the kicking team blocks on the receiving team's 30, it's a foul. A "regular" block would just be a 5 yard penalty, though; not sure what would cause that to be a 15 yard unless it was targeting, BBW, etc. In the case of the player who recovered the ball while on his knees, perhaps the foul was for a late hit? Like JasonTX said, really hard to say without seeing the play.