The whole team fucked up, but it boiled down to a key issue. Player hid thus allowing Southampton to win the number game in key area. Not in this one instance only, but also throughout the game.
Harry Winks casually with the least effort jogged into no man land. There was no potential of any passes into this area, nor no Southampton players to be marked. Winks "played" himself out of the tactic board here. This gave Southampton tactical advantage to break our shape apart. Ben Davies was pulled from the back line by Amstrong who moved away from our box to provide a quick short pass option for KWP and JWP if they needed. Romero reasonably had to move in front of his mark to cover for a low cross to the near post. This further dragged Sanchez toward near post and isolated Emerson Royal. That's the bigger picture tactical issue here.
Then we have other issues like Kulu irresponsibly failing to track his man; Son had become very lazy tracking back, applying pressure (JWP should be pressed by Son); Sanchez and Emerson Royal don't know how to play an offside trap despite Lloris constantly commanding them to "Get out" . If they played the offside trap right, they wouldn't have to screen behind those offsided players, and Emerson Royal would have a better chance to notice the goalscorer sooner thus a better chance to compete for that cross and prevent the goal. In comparison, see how Southampton was so disciplined with their line in Bergwjin offside goal or Kane being fouled in offside position? They're well drilled to keep their offside trap till the end of the playing phase. Every detail counts at this level.