#3 Too many errors from the Spurs
Spurs under Pochettino likes to keep their balance and stability. They were the exact opposite at Emirates. They lost possession a lot in the first half hour, completing just one or two passes before the ball was pinched. As a result, Arsenal burst through the sides and caught Spurs off guard.
During a free-kick, Arsenal caught Spurs half asleep, and a quickly taken lobbed freekick found Iwobi under no pressure. Spurs lived dangerously due to missed chances and poor finishing by Arsenal in the first half.
Jan Vertonghen's poor reading of the corner was a huge error in giving Arsenal the first penalty. No one pressed Hector Bellerin, allowing him to take all the time to weave a through ball for Ramsey. Ramsey and Aubameyang were under little pressure from their markers Vertonghen and Dier.
Juan Foyth gave the ball away cheaply, which allowed Lacazette to take a shot away. Then Hugo Lloris' complacency let the ball slip in. Dier tried to win the ball when Torreira was through, he ended by missing the ball and letting Torreira slip by.
Three clear errors led to goals, and an awful lot more mistakes turned the game into a nightmare. Spurs just made too many errors.