"Did they reach a high enough altitude for the (lack of) pressurisation to trigger the oxygen masks, or did the pilots activate the masks - if so, why?"
My Question also. Seems strange, especially if they went to just 3000 feet to prepare for a go-around. Or did they climb beyond 10,000 feet and trigger deployment when they depressurized as per the check list??
Seems they were in the air for 20 mins. A bit lengthy for a go-around?