It all stems from them being all too happy to play it backwards. He could have easily played it forwards instead but did a tentative shitty pass to Leno.Bench Cam from the game against the ChavRentBoys
The look on Arteta's face (He Freezes and his face says WTF!!!???) when mustafi makes THAT mistake.
Play forwards not backwards and you never give that type of goal away, even if it’s a hopeful punt to the corners, to turn their defence, give Martinelli something to chase.