I think the answer may be that you got hit by a late frost this year.
From the RHS advice site:
Walnuts grown for fruiting are types of the English or Persian walnut, Juglans regia. Some common cultivars include ?Broadview?, ?Buccaneer?, ?Franquette? and ?Northdown Clawnut?.
They can either be grown as a standard with a central leader or kept small by removing the leader. Central leader standards should have their lowest branches removed back to the trunk while still young. To keep both forms of tree relatively compact and bushy, the new growth of the side branches can be pinched out at the fifth or sixth leaf every summer. Otherwise, where space is not limited the tree can be allowed to grow unhindered with pruning kept to a minimum.
Walnuts grown purely as ornamental trees do best as central leader standards with the lowest branches removed in the early years. If the leader is lost, perhaps due to frost, it should be pruned back into healthy wood in summer and a sideshoot used as replacement. The black walnut, Juglans nigra is less inclined to lose its leader than J. regia, J. cinerea and J. ailanthifolia.
Pruning
Established walnuts, whether fruiting or ornamental, are best left unpruned. Where pruning is necessary it is important that the work be undertaken between mid-summer and early autumn. Prune side branches to a main branch or to the trunk, cutting along the line of the branch collar (the swelling where one branch joins another or the trunk). This promotes healing and prevents decay entering the trunk. Hard pruning is not tolerated.
Avoid pruning walnuts in late winter or early spring (January-April) as the sap rises early and the pruning cuts will bleed profusely.
Pollination
Walnuts are wind-pollinated. Most are self-fertile, but the flowers open in early spring, making them vulnerable to frosts. On individual trees male flowers open before females so cross-pollination is more successful with at least one other tree within about 80m (27ft).