In your first article, Captain Sabharwal, the expert cited, points out that the something I imagine Captain Steve, the expert in serendipity's article, saw as well, namely that the landing gear hadn't been retracted. Mr. Sabharwal does mention Steve's theory, though he thinks it's unlikely:
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"And the third, very unlikely scenario, is that when the captain says 'gear up', the first officer or the co-pilot, instead of raising the gear, retracts the flaps. The retraction of flaps will result in immediate loss of lift. And with the gear down, the thrust will not be able to cope up with the drag being created," Mr Rai told NDTV. "The aircraft will not be able to climb."
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Here's Sabharwal's top 2 theories:
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According to Mr Rai, several scenarios could have happened. One could be that a bird hit has taken place and both pilots have been taken aback, and in the process they have forgotten to raise the undercarriage. However, this [bird hit] has been ruled out by many aviation experts.
Another scenario Mr Rai explained was the possibility of a sudden loss of power, which may have distracted both the pilots, and in the process the undercarriage remained in landing position.
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There's a paywall on your second article, all I saw was that there were various theories, but not what those theories were.