Looks like it may be getting too much water. Hardy garden euphorbias are pretty water-shy plants, requiring only enough to keep hydrated. Excess water - which is very easy to over-do in a container situation - will result in the fellowing and dropping of foliage you see here.

Also (and not sure this is applicable in this situation), the old flowering stalks of these euphorbs will eventually deteriorate and the foliage will yellow, dry up and fall off. These stems should be removed at the base of the plant once the flowering period is over. Be sure to use hand and face protection when cutting back euphorbs, as all release an very caustic sap (a milky latex) when cut that is very irritating to many individuals and can cause harm if it comes into contact with any mucous membranes (eyes, nose, mouth, etc).