A Police spokesperson said that this action was taken due to "quite a lot of anti-social cycling in the area." When asked to confirm what anti-social cycling was they said it applied to those who were riding more than two abreast, or in big groups, and causing cars to slow down.
Rule 66 of the Highway Code states: never ride more than two abreast, and ride in single file on narrow or busy roads and when riding round bends.
Cycling in groups is not restricted in any way.