It could be the spray. Weed-B-Gone is a growth stimulant. In mild doses it causes misshapen growth. In high doses is stimulates fatal amount of growth.

Some other possibilities are:

Heat: Leaf roll and droop is normal when rhododendrons are exposed to extreme heat, extreme cold, or drought. It is a natural reaction that helps prevent desiccation under these conditions of extreme stress. The roll and droop should go away when the extreme condition is absent. Since the same condition can be caused by root fungi which thrive in warm moist conditions, it is important not to over-water. If the soil is truly dry, then watering approximately once a week in normal soils may be helpful. Sandy soils require more irrigation. The soil should not remain wet.

Aphids: Distorted young leaves covered with aphids can be a problem in the spring. These small sucking insects feed on the new growth as it begins to expand causing distorted leaf shapes. In large number, their sucking damage causes new leaves to twist and curl. A secondary problem is aphids excrete a sugar solution called "honeydew"�a perfect food source for the sooty mould fungus which grows on the honeydew causing plant leaves to look like they are covered in black soot. The fungus isn�t actually parasitizing plant tissue, but it coats the leaves to such an extent that plants can�t photosynthesize efficiently. In time, natural predators will help. The most famous is the ladybug, whose larval stages eat quantities of aphids. There are other predators, too. As a last resort, insecticides are available. The aphids can be controlled with Orthene, Malathion or Diazinon.

Leaf Miners: Skeletonization areas near midrib and leaf curling is indicative of Azalea leafminers, Caloptilia azaleella. Mines and skeletonization are initially formed near the midrib. As larvae mature they curl the tips of the leaves with silk and feed inside the curl.Large populations cause leaves to brown and drop prematurely. Curled leaf tips in June indicate the completion of the first generation. The second generation mines begin in July. Adult moths are present in late June and August. The moths are 3/8 inch long and yellowish brown. Mature larvae are 1/2 inch long and yellowish brown. Azalea leafminer overwinters as pupae in leaf mines. Rake and destroy fallen leaves in the fall to remove overwintering pupae. Infested plants may be sprayed with a registered insecticide in May if many developing blotch mines are observed. Evaluate the plants in July to determine if a second treatment is necessary.

Midge: Young leaves rolled and distorted could be rhododendron gall midge, Clinodiplosis rhododendri, which are 3 or 4 times larger than a grain of Salt. The rhododendron gall midge is a tiny insect that feeds in the spring and causes distorted leaves to emerge. There is no way to undo the damage once it has occurred.