According to the DPCC, the reduced pollution level was due to lower car usage, which in turn reduced traffic on the roads.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/car-free-zone-sees-%E2%80%98significant%E2%80%99-drop-pollution
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/indian-express-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/car-free-days
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi-pollution-control-committee-dpcc
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/centre-science-and-environment-cse
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/lung-disease
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/child-health
[11] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects