When the ball is on the LW, where it was, the LB for us tucks in. This is basic football. The opposition RB on the touchline can only hurt you if you make the ball to him easily.
To say that Siggy was only bought for offensive reasons like
@trueblue10 is insinuating is tosh. A large proportion of the game will be spent without the ball, especially away, so he needs to be able to defend.
But he didn't. He made the pass a piece of cake and forced Martina into a decision. The distance between him and Aurier is huge which means he's caught in two minds as to whether he will get there in time. Instead he stays and hopes to guard against a tap in from Kane et al. He didn't do this well because his body position is wrong but the theory of his thoughts was understandable. You can visibly see he is caught in two minds.
We basically allowed Spurs to do to us what they did at Goodison. Overload one side, quick switch and hope to isolate our full back.
It's a mixture of Allardyce / Siggy / Martina. Not the latter alone.