Well, since you and 'Bike' asked, take your pick from the below.

1. This kind of 'trash' has been 'news' on LI many times in the past and for good reason. Arguably, Sandip Bhagat's move from Delhi to Mumbai in 2010, and Rajat Sethi's move in 2014, made S&R the firm it is today.
www.legallyindia.com/law-firms/sandip-bhagat-will-move-to-mumbai-to-set-up-new-sar-office-20100702-1053
www.legallyindia.com/the-bench-and-the-bar/s-r-rajat-sethi-follows-bhagat-moves-from-delhi-to-mumbai-in-corp-drive-20141107-5279

While much more political at Amarchand in the old days, moves between offices were fairly significant within the legal ecosystem back then, such as VP Singh's and others: www.legallyindia.com/law-firms/vp-singh-moves-from-amarchand-delhi-to-mumbai-20130410-3583

You could also argue, that Manan Lahoty's sort-of move to Mumbai around 2009, made Luthra's cap markets practice as successful as it is today.

And there are many more examples, but I don't want to bore you.

2. As far as I'm aware, Karan Singh has basically spent his entire Trilegal career in Mumbai, playing a huge hand in building up that office to what it is today. The move may be for personal or lifestyle reasons, but law firms are made of people, and if a founder or senior partner moves, it will change the DNA and culture of an office, at least a little bit.

3. For the trade press, such as LI, or The Lawyer, or Law.com, etc, it is 'news' in foreign legal markets too.

4. I believe, for those that didn't know yet in the Bangalore market, this would be 'news' too. Having one more senior yet young-ish corporate lawyer in Bangalore could mix things up a fair bit there. Bangalore's become quite a settled market, carved up between the big firms, I feel. So it's quite possible, there's also a strategic dimension to the move, to boost the corporate presence there amongst the Flipkarts of the city.

5. It has to be news, because otherwise it'd make me wonder, why did you click on the story and comment on it?

6. It's news, because it's information that wasn't public before and that you (presumably) didn't know about. It may not be news you personally care about, but that doesn't stop it from being news for someone else.

Hope that clarifies. Yours, with massala,
Kian

I actually don't know any specific further details re this specific case, at the moment. I'm a bit biased, because I'm also strongly considering moving there, but I would expect that it's partly a lifestyle choice because of excellent Bangalore weather and beer :). And in Karan's case, there's probably also a decent business case to make, since the Mumbai office is much bigger than Bangalore.