I'd say the big thing is they took their core product as granted, and focused a great deal of their income on almost anything else, aiming/hoping for some horizontal growth instead of investing to preserve their processor market share. Intel is flush with cash and could either invest in CPUs, or, say, buy McAfee, a brand that had lost most of it's value a decade prior. Or maybe acquire some HPC products to try to build an in-house all-in-one HPC solution to compete with their partners, then decide that was a bad idea and mostly abandon that expensive effort. Or maybe buy an ethernet switch chip company, and then promptly do nothing with it. Since it worked out so swimmingly the first time, do the exact same thing with another ethernet switch chip company and again just shrug and never do anything with it. Maybe spend a boat load of money trying to make "Optane" a thing, including heavy evangelizing to try to convince people to fundamentally rework core concepts of how they work to justify the apparently awkward in-between of PCM which was never going to be as fast as SDRAM nor as cheap as NAND. Along the way spend money on all sorts of weird random projects someone had without any target customer expressing interest in the hopes they stumble upon some unexpected Model T moment in a new market segment.
Intel just assumed their position in the market was unassailable and went about trying to start *something* else because protecting their core business wouldn't deliver adequate growth (they pretty much had the market cornered). So you have a lot of big 'lottery ticket' investments with inconsistent execution on top of dubious justifications in the first place. They failed to coalesce around a common accelerator/GPU strategy leaving them critically disadvantaged compared to AMD and nVidia, their CPUs surpassed by AMD, their fabs long passed by TSMC and none of their gambles paid off, so now they are just boned. I suppose on the upside, *now* they have growth opportunity in their core competency since they ceded so much ground...