Ceanothus is a short lived shrub. We are on our fourth one in the same border. The severe winter and dry/hot summer may have stressed it out too much. Our first ceanothus got very large, was loosened in the ground by a severe gale and died rapidly. The other 2 got brown patches and died gradually. This border gets a lot of sun and is dominated by a Hawthorn, so may be the same as your Oak, taking up moisture, and now struggling itself in this dry hot spell.