I imagine someone in Derbyshire Police with some time on their hands (perhaps doing light duties due to an injury or something) has been given the job of trawling through firearms certificate holders and matching them against incidents which they have had some involvement in and come up with a list. They've then made a "hit list" of folk who have displayed actions that might call their suitability into question, or have suffered from a mental condition and then done the rounds.
However, most forces automatically do this whenever you come to police notice. If, for example you get arrested for something or are engaged in some sort of disorder, domestic incident, drink driving or whatever, as a matter of course, it is flagged and a decision is made concerning your suitability to possess firearms.
I can only surmise that Derbyshire hasn't been doing this, they've realised this (or had it pointed out) and have mounted an operation to rectify this. So whilst it's perhaps normal to think Derbyshire are getting tough with certificate holders, the real worry might well be that they've been allowing people to have guns who simply aren't suitable. So not quite sure why Derbyshire are boasting about this. Polishing a t*rd perhaps.