So a combination of low overall experience, low type experience, SOPs that have the PM (was the PM before being the PF before being the PM !!!) instinctively looking down at a critical moment of flight contrived to catch the FO out.
Airbus SOPs have the PF always in control of the thrust levers, including non-normal events. In the days of turbo-props with flight pitch locks etc, the PM may have needed to be involved but in modern jets this is ridiculous!
If, as stated above, BA got rid of the pilot then shame on them. Where were BALPA?