It's propaganda pushed by right wing Hindu nationalists. You might be seeing more articles about it due to Indian politics over the last decade. Hindutva, a movement going back to the late 19th century came into mainstream with the election of the current prime minister in 2014.

The term "Hindutva" was popularized in 1923 by V.D. Savarkar who greatly admired Adolf Hitler and shared many sentiments with him. Both Savarkar and M.S. Golwalkar, one of the early leaders of the paramilitary wing of Hindutva, RSS, explicitly wrote that Hindus should use the Holocaust as a model to treat Indian's Muslims. RSS has always been characterized by wanton violence and brutality; it was an RSS operative, Nathuram Godse, who assassinated Gandhi.

Hindutva ideology involves a bizarre understanding of history. It holds that India has had two eras. A golden age of Hindu rule where science and technology flourished. This was followed by a dark age, where Muslim invaders entered into India and ended the golden age. To support this understanding of history, any and all positive aspects of Indian history have to be revised to ensure that it is attributed to Hindus.

This specific "theory", if we're going to grace it with that term, was propagated by a historical revisionist named Purushottam Nagesh Oak, more commonly referred to as P. N. Oak. It wasn't just the Taj Mahal that he claimed was a Hindu temple. He also said that the Vatican, the Kab'a, and even Westminster Abbey were originally temples dedicated to Shiva. I believe he also considered Abraham, Moses, and Jesus to have been Hindu priests. The "theory" would have been lost amidst the detritus of similar conspiracies if it weren't for Hindutva and the politics I mentioned earlier, bringing it into the mainstream through repeated court cases in India.

This movement has directly impacted historians who study Indian history. I want to keep my answer confined to >20 years ago, but you can google "Audrey Truschke" and see the backlash, harassment, and threats an accomplished historian has faced just for writing a well-researched historical book about a person who has been dead for centuries. She talks about the difficult decision to allow her book to be censored in order to have it published in India. And although nowhere near the same level of harassment that she's received, I myself have received threatening PM's for reddit comments I've made about Indian history (and have since taken to periodically deleting my post history to minimize the chance of doxxing).

For further reading, I would recommend a very recent article by Audrey Truschke, Hindutva’s Dangerous Rewriting of History. It was published just a week or two ago.