Health experts have attributed the rise to a possible "evolution" in the virus that carries this disease and change in weather factors like humidity.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/chikanguniya-cases-rise-new-delhi
[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/chikungunya
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vector-borne-diseases
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/diseases
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health