Disclaimer - this is a purely speculative answer, but in the absence of anything else, it may be worth considering.
The Hindu Rakshasa legends feature them in a huge variety of forms - often multi-headed, animal headed, ogre-like or Other. Kolchak's rakshasa was also something like a 'clawed sasquatch', not a tiger. Yet Gygax rakshasas are traditionally 'humanoid tigers' in their natural form.
I suspect this is because of "tigers themselves are regarded as sorcerors in India" legends, so they were a natural pick for something that could be reasonably illustrated and consistent, especially with the sorcery tie. This is admittedly a bit flimsy based on a Wikipedia article for Tigers citing an 1910 Encyclopedia Britannica that I don't have access to ;) There's also at least one rakshasa known for having a tiger head (possibly Gygax just 'went with that one')
However, look further into general Tiger-lore in Asia and you start to come across WereTigers... tigers shape changing into human form, using sorcery and other D&D-like rakshasas traits. So this makes me think that if Gygax was combining another mythological source into his 'final' D&D rakshasa, he might have been adding a blend of Asian style weretiger legends. (especially if he was inspired by the Kolchak episode that drops the rakshasa with a blessed crossbow bolt - pretty similar to a silver bullet)
Even more interesting, you start to see things like 'you can tell a were-tiger from a real tiger if it has 4 claws instead of 5'. That's not quite 'reversed palms', but its starting to get warm. Moreover, some of the lore has Tigers that start to become human after they have eaten a number of humans. I don't have a definitive source that says 'and reversed palms is one of their tells' but it might be out there.
There are at least a couple of other full books specifically devoted to Asian/Indian weretiger lore (unfortunately not free). But they may confirm the suspicion or provide you with some more interesting and related source material.
Tracking the Weretiger - Supernatural Man-Eaters of India, China and Southeast Asia
By Patrick Newman
By Shaiontoni Bose, Arundhati Dasgupta, Bunny Gupta
Another book that Gygax himself might have read which contains some Rakshasa material is Roger Zelazny's "Lord of Light". In Appendix N of the 1st edition Dungeon Master's Guide, Gygax cites Zelazny under Inspirational Reading. Though he doesn't give this book in particular, it was written in 1967 and it weaves Hindu mythology in a science fiction/fantasy setting.