Find out why a NPM package is installed

npm why <package

For example, running npm explain glob can output something like:

glob@7.1.6
node_modules/glob
  glob@"^7.1.4" from the root project

glob@7.1.1 dev
node_modules/tacks/node_modules/glob
  glob@"^7.0.5" from rimraf@2.6.2
  node_modules/tacks/node_modules/rimraf
    rimraf@"^2.6.2" from tacks@1.3.0
    node_modules/tacks
      dev tacks@"^1.3.0" from the root project