You would hope that his finger was nowhere near the trigger, so perhaps some part of his clothing or the lanyard got caught in the trigger guard.
That would not be unheard of but very unlikely to operate the trigger mechanism!.
At the end of the day the UK POLICE are among the best most well trained officers
And safety is a lot higher on the agenda in UK firearms police training than in other countries
Especially with regard to officer safety and the public safety but
"Shit happens" and know one got "seriously" hurt hoping the officer recovers soon because he'll never live it down!