His tariff had expired but due to the type of sentence he was on, instead of having a parole hearing when it expired it was delayed by 18+ months.
The judgement refers everyone's right to fair detention, set by a court, and because of the workload of the parole boards, due to these intermediate sentences, they decided that had been breeched.
HMG have shelled out £300k+ so far so it's very unusual for ECHR to get involved.