This is not a new thing. I think they have been using trials bikes for over ten years now.
The first time I saw them was on the Four Bridges of all places. As they came closer I could see on the flyscreen above the headlight was the word 'POLICE' but they caught my attention for another reason (better not say what!) Note that the police also have quad bikes.
The fact that you don't see them very often is, I hope, an indication that they are being deployed where best needed.