In many parts of the US it has been a wetter spring then usual. This can cause leaf spot diseases. Aphids can also cause yellowing of leaves. From this site:
Sunken Spots On Leaves
Anthracnose - This fungus disease causes distinct lesions on maple leaves, which appear as moist, sunken spots with fruiting bodies in the center. The leaf spots may run together, causing a blotch or blight. The dead areas follow the veins or are bounded by larger veins. Sometimes the ends of young shoots blight down to several inches. Pustules containing pinkish spores appear. Dieback and loss of foliage may occur in severe cases.
Gather and destroy diseased leaves when they fall to prevent the spread of the fungus. Replace existing mulch with fresh that does not have fungal spores on it. Prune away diseased branches, then disinfect your pruning saw or loppers by dipping them into a solution of hot water and household bleach. Maintain tree vigor by feeding and watering it well, especially during droughts. Mature trees can usually handle this disease and regenerate new leaves.
Leaves Wrinkled, Stunted, Defoliation
Aphids - Aphids are soft-bodied and pear-shaped, about the size of a pinhead. They may be green, brown, or pink. They retard or distort tree growth, especially on young trees. Maple leaves may turn yellow or brown, wilt under bright sunlight, or curl and pucker. They may actually drop off the tree. Aphids suck plant sap from the leaves and excrete drops of honeydew. With heavy infestations it can be quite messy, although studies have shown that these abundant sugar secretions, soaking into the root zone, actually benefit the tree.
Foliage Turns Yellow Or Brown & Collapses
Verticillium Wilt - A wilt disease caused by a soil-dwelling fungus sometimes attacks maples. Infected leaves appear pale and wilted, and may fall prematurely. One or more branches wilt suddenly and die, often on only one side of the tree. Infected trees may die slowly (over a period of several years) or suddenly (within a few weeks). Sapwood is discolored. Heavy feeding with a high-nitrogen fertilizer sometimes enables trees to put a new ring of sapwood outside the infected area and the trees may then recover. Prune out dead branches. Remove badly infected trees, together with as many roots as possible. Do not replant with wilt-susceptible shrubs or trees in the same location.
This site also lists some problems, but spider mites also come to mind.
Spider mites are another insect that can also feed on Japanese maple trees, but are harder to detect. These tiny insects are about the size of a pin head. Spider mites reproduce rapidly during hot dry weather and live on the under sides of leaves this makes them tough to eradicate. The damage from spider mites is hard to detect by the untrained eye. If the mature leaves appear appear speckled or faded in color, I recommend checkong for spider mites by placing a white piece of paper under the leaves and tap the leaves vigorously. This method will knock the mites from the leaves and onto the paper. Next, take a very close look at the paper, spider mites will be reddish colored or light colored with two brown spots on their back. Spider mites are sucking insects and drain the leaves of vital juices needed to keep the cells turgid and alive.
From this site:
Phyllosticta leaf spot
Phyllosticta Leaf Spot of Maple. Pathogen: Phyllosticta minima
Like many fungal leaf spots, this disease affects a number of maple species, most prominently Amur, Japanese, red and silver maple. The disease causes little damage because the infection is localized. Spots are roughly circular and develop into tannish spots with purple to red borders. Later in the season the spots often contain black fruiting bodies of the fungus arranged in rings inside the lesion. Although this disease is quite noticeable in the landscape, especially on silver and red maples, and causes concern among homeowners, damage is minimal and fungicides are rarely necessary.
You can, if there are a few infected leaves, just pick these leaves off and put them in a plastic bag and throw them in the trash can. This will cut down on the amount of spores that would infect other leaves. But the fungicide will work best. This is not a serious problem it is mostly just cosmetic and looks a lot worse then it really is. It will not kill you tree but will make it look bad.
Since neither of you mention wilting of the leaves, I didn't add info about Verticillium wilt, but the sites I included have info on this disease.
Does any of that sound or look familiar with what you are seeing?
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.