There is a legal process they must have gone through.
Admission authorities of maintained schools must set admission numbers with regard to the capacity assessment for the school . Admission authorities may fix an admission number for a relevant age group that is lower than the capacity assessment. If they do so, they must publish this information for parents at the same time as they notify the bodies they are required to consult about the determination of their admission arrangements, who may object to the admission number.
I don't know what the reason for the change would have been in this particular instance, but there must have been one. There may be an explanation in the case papers, but questions can certainly be asked about it at stage 1.