Collapsing maul, YC but no PT?

[pombok]

Gloucester v Exeter last night. Exeter form a maul and keep it driving towards the line. 5m out and Tuisue collapses the maul. Ref Adam Leal YC Tuisue and awards Exeter a penalty. He says there is no PT because the ball was at the front and it was 5m out. What is the logic here, that Tuisue was entitled to tackle the ball carrier? Why no PT if he collapsed the maul?

 

[Decorily]

Didn't see it but possibly the referee was of the opinion that even if the maul had reached the goal line there were defenders in contact with the ball to prevent a grounding..."Ball at front".