I'm not a super expert, this is just what my friends in the biz have told me. I wouldn't be surprised if several factors conspired to create the IPA trend. Yes, there are many ways to infuse flavor into beer, some of which you mentioned above. My friends make IPA styles, but also many others using all kinds of tricks like malts, waters with different minerals, pHs, different base ingredients, brewing wheat in different ways. My buddy even domesticated a new strain of yeast.

If you're in Boston, give Aeronaut Brewery in Somerville a try (https://goo.gl/maps/bm22XaBvFPQ2). I have something of an interest of it, but I (unsurprisingly) recommend them.