Q:

What must be causing a decrease in sparrows and an increase in pigeons in my town?

Has your city seen a noticeable decrease in population of common sparrows? And, on the contrary an increased population of common pigeons?
As per my opinion prime suspects appear to be stronger cellular networks and pollution?
Details: 
City - Mumbai and Pune.
Country - India.
Continent - Asia.

A:

Common Sparrows, which are the State Bird of Delhi, and also of Bihar, have indeed been on a massive decline in India in recent years. What you're seeing is not a fluke. It's happening in many urban areas in India, and is a serious problem. 
Although many studies have been done over the years, the largest group in your area which is currently involved in the study and preservation of sparrows is The Nature Forever Society. 
Since 2005, they've been working for the conservation of house sparrows and other common flora and fauna in urban habitats. Their mission is to involve the citizens in the conservation movement of India, especially in urban landscapes.  I recommend spending a lot of time on their website. There's a tremendous amount to learn, and things you can do to become involved. For instance, each year on March 20, they sponsor World Sparrow Day. They also give out Sparrow Awards to regular citizens who are making a real impact in the lives of sparrows.
Their research is the focus of an article published in March of 2016 in the Daily News Analysis, entitled 10 reasons why the sparrow is fast disappearing from Mumbai.
In the words of the author Shraddha Shirodkar:

While no one can be singled out for the declining sparrow population, it is humans who are collectively responsible for it, explains Mohammed Dilawar, President, Nature Forever Society (NFS), a crusader for the sparrow as well as other common flora and fauna in urban habitats. Through his non-governmental, non-profit organisation, he has been championing the cause of sparrows, by involving citizens in the conservation movement, especially in urban areas.

She compiled a list called "10 reasons why Mumbaikars need to save the sparrow". I removed the tenth because I found conflicting evidence from other sources. The bullet points are hers, but also include information from other sources that said the same thing. 

Felling of trees
It is common knowledge that more the number of trees, more the number of birds. The spike in the felling of trees in Mumbai is a major reason why sparrows and other birds are facing a loss of habitat, especially their natural nesting spots. 
Lack of cavity nesting
The ubiquitous glass buildings of Mumbai—the corporate dens—have replaced many older structures that were built with a façade that had nooks and crannies, even bricked roofs, which allow sparrows to nest. 
Absence of native plants
Native plants such as adulsa, mehndi and many others are outdone by fancy non-native ones like Duranta Erecta, Dumb Cane and others as the trend of modern landscaping catches on. Native plants are the natural habitats of sparrows, providing them insects such as aphids to feed on. Sparrows need a diet of insects in their formative years to grow into healthy adults.
Absence of hedgerows
Contemporary landscaping is also doing away with hedges, which are preferred by sparrows for nesting. Thick hedgerows are known to protect nesting birds such as the sparrow from predation.
Widespread use of concrete
Sparrows are known to take two types of bath—one with water and one with dust. With the extensive use of concrete in Mumbai, the species is unable to take dust baths.
Modern grocery storage
Sparrows are known to feed on tiny grains like bajra, which used to be freely available from pecking at gunny bags stored outside older-style grocery stores. They were able to spill seeds on the ground. Also, grocers and street vendors purposely dropped grain and seeds to encourage the sparrows. Modern grocery stores with air-conditioning and plastic packaging take away any chance of finding food grains to feed on.  
Chemical fertilisers in agricultural produce
Heavy use of chemical fertilisers leads to agricultural produce being laced by them, hence ruining the food of sparrows. 
Cell phone radiation
The electromagnetic fields and radiation created by mobile towers are known to affect sparrows. The effects range from damage to the immune and nervous system of sparrows to interference with their navigating sensors. They tend to get confused and leave the area within a week. Since the eggs they've laid nearby need two weeks to incubate, they don't end up hatching. (I found this in many publications, as it's a very common theory at the moment.)
High litter index in Mumbai
There is a rise in the population of crows and stray cats due to the high litter index in Mumbai. Simply put, more the garbage, more the predators that prey on sparrows.

Additional sources for interesting information:  
Disappearing Sparrows:Common Bird Goes Uncommon
House sparrow listed as endangered species
Citizen Sparrow. This is an interactive site with opportunities to help keep track of sparrows. It has reports of sparrow sightings all over India, with maps and other fun stuff for sparrow-lovers!
Sparrows of India

A:

At a guess?  A change in architecture.
Pigeons favor cliff ledges as nesting sites; they'll happily build nests on window ledges as a substitute for natural cliffs.  Sparrows favor cavities: tree hollows, the eaves of roofs, and other somewhat-enclosed areas.  If there's been a trend of replacing traditional houses with high-rise apartments, I'd expect a corresponding shift from sparrows to pigeons.

