So this is an odd one which no one I've asked can actually answer. In all scenarios the second row immediately turns to face his teammates.
Scenario 1
The second row catches the ball from a lineout. The opposition grab him. His forwards form around him and the ball is smuggled to the back. A maul is formed.
Scenario 2
The second row catches the ball from a lineout. The opposition remain a yard back from him. His forwards form around him and the ball is smuggled to the back. They then maul forward into the opposition and the referee pings them for "Truck and Trailer".
These two scenarios are relatively common and are understood.
Scenario 3 (As seen during the recent opening round of the rugby World Cup)
The second row catches the ball from a lineout. The opposition remain a yard back from him. His forwards form around him. Recognising that he has not been tackled he holds on to the ball and, while making eye contact with the ref, starts shouting "I still have the ball". Avoiding the previous potential for offside (accidental or otherwise) the players begin to trundle forward. One of the defenders now performs a chop tackle on the second row who still has the ball, taking him down and, by proxy, a number of the other forwards driving in. The referee, in this game, immediately blew for an infringement by the defender.
My question is, how is this "bringing down a maul" if the maul has not yet been formed. Surely this is no different to a player running at the defensive line with a support runner driving into the defenders alongside?