Some bumps on leafs are called galls and are caused by insects.

They happen when a tiny organism (like a mite, insect, or fungus) feeds on or lays eggs in a developing leaf. The plant reacts to the chemical irritation by rapidly growing abnormal tissue around the invader—essentially building a little protective room for it.

​Leaf Galls are typically the same color as the leaf or turn bright red, yellow, or brown: Mites, aphids, or tiny wasps can cause them
The good news is they rarely harm the overall health of an established plant or tree.