The explanation given for frothing of Bellandur and Varthur lakes was inlet of raw domestic sewage with high concentration of phosphates used in detergents.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/it%E2%80%99s-indication-rising-pollution-level%E2%80%99
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/hindu-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/karnataka
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/lake-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/bangalore-t
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/environmentalists
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/sewage-pollution
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/water-pollution