Spindle galls are a plant's reaction to tiny mites, Eriophyid mites to be exact. When they feed on a leaf it triggers the reaction and the leaf creates a growth that encapsulates the mite. The mites take advantage of this growth and use it for shelter, food and a safe place to lay eggs.
These microscopic mites lie dormant in the winter in tree bark, leaf buds and leaf litter. Once trees leaf out, they begin feeding.
There's no need to worry, except for a few ugly looking leaves, they are not known to be any serious health risk to your trees.