Why do some powders clump in hot water?
Water is a great solvent for polar molecules. Sugar, table salt, and other small polar molecules are water soluble. When you put them into water, you get a sugar resp. salt solution. 

Other molecules are not soluble in water. Most organic molecules with a carbohydrate tail are insoluble (unless they have a strongly polar active group, like the shorter alcohols). Starch and cellulose are not water soluble. Most plant matter is made mainly from starch and cellulose. (The exception are fruits, which can have more fructose than starch, but still lots of cellulose). 

When you put non-soluble powder into water, you are not making a solution. You are making a colloid known as sol (solids dispersed in a liquid). If you wet salt, the water will dissolve the first layer, and the second layer will be exposed. If you wet a random non-soluble powder, the water will have to wash away the first layer before it reaches the second layer, so it is hard to make the suspension without vigorous stirring. 

There are some special molecules which absorb water and swell. Starch, some proteins (gelatin) and some polysaccharides (xanthan gum) are common such molecules in the kitchen. When you wet them, the water gets absorbed by the first layer. This changes the viscosity of the layer, making it thick and clingy. This forms a clump. In the core are the dry layers of powder. They are held together by a bubble of the water-swelled substance. Outside of this bubble is the water. It can't wash away the bubble, because it is so viscous, it doesn't flow into the water. And the bubble protects the core of the clump from contact with water. This happens with starchy powders like flour, or also real cocoa powder. Tea powder is mostly dried cellulose, so it is also hard to dissolve. 

You mention cocoa as easily solved in water. In fact, cocoa is very hard to dissolve, so I think you are speaking of a cocoa flavored drink. A cocoa flavored drink is a mixture of dextrose, cocoa powder, and other things (but mostly dextrose). In such a mixed powder, the water dissolves the soluble molecules (the dextrose), and the insoluble particles (the cocoa powder) which were suspended in the dextrose are now suspended in the water. No neighbouring cocoa particles to cling to. So, no clumps, but an instantly homogenous drink.